From testing (waaay back) when I first got my old system running and some separate affirmation with newer NPK testing there were several "knowns".
-that running solely a high urea / low nitrate fertilizer throughout an entire flower cycle does not have a good chance for success; without significant issues.
-that running solely a high urea / low nitrate fertilizer in early bloom is likely to result in larf / popcorn issues (too large of node spacing) along with Ca- and Mg- issues.
-that running solely a high urea / low nitrate fertilizer in late bloom, is likely to result in foxtailing and/or poor taste.

-that since changes to the N component of NPK ratios in early and late flowering produce negative results; positive effects must have occured during peak bloom (i.e. when considering yield change from old vs new methods).
-that changes to the N component of NPK ratios in peak flower had the largest impact on N toxicity, but the lowest impact on larf / popcorn (when compared to early or late flower).

For the sake of accuracy. I finally ran 1 test of my bull "as I had before" when I did things "wrong" (During the flower period). The yield from this was increased above all other NPK or PK testing and was within a "reasonable percentage" of what I used to get.
*Note: (In my environment / setup) using "hydro" nutes, the NPK maximized @ 126-68-215 had a max avg. yield of +/- 30 with bull. The above stated first test run with the SAME NPK @ 126-61-207 (wk 3-9) and using urea; resulted in a "decent increase"

1. This formula is ONLY to be applied starting at the beggining of peak flowering (I.e. floral meristem). It should not be started until "Stretch" is complete, but before "stacking" has started.
2. The fomula is applied for appx 3 weeks (for 9-10wk strain) after starting. (I.e. in typical place of a pk booster).
3. Only add enough water for 24hrs maximum during this time. Run-off during this period is not necessary or recommended.
4. After the 3 week application (as well as before in early flower*) any choice of "normal" nutrients high in Nitrate (and Ca, but no Urea) can and should be used.(I.e. whatever your using now).
5. Optionally and would be recommended, is to go into a true PK boost immediately following the Urea application period to asist with expedient swelling of calyx.
**High soil Ca levels are Important going into the application of this formula. Some strains that demand higher Ca or Mg "may" require foliar application during this time.
*This is intended for soiless applications, but "Some" organic component is recommended (ewc, compost, humus etc). If Cap's benny "Nute pack" has bacteria for Urea; that would work as well.

-For those that Mix raw salts: I've been utilizing appx 40% Urea - 40% Nitrate - 20% Ammonium for the formula and would consider that a "good starting point".

-For those that may wish to try this without Miracle-gro; I can get a profile put together for this.

-For those who may wish to try this out possibly (or want to see) what I am using; the formula (per gallon) is:
1.7gm Miracle-Gro All-purpose 24-8-16 (urea component)
0.5gm Better-gro Orchid Plus Bloom 11-35-15
1.4gm General Hydroponics Maxibloom 5-15-14
0.8gm Potassium Sulfate 0-0-50 (not 0-0-52)
0.5gm Calcium Nitrate 0-0-15
0.3gm Magnesium NItrate 11-0-0 (9.6%mg)
Additive
1.0Ml Floralicious Plus (non-critical)

(NPK,Ca,Mg = 126-61-207-42-21 Total Cost of base is 4 cents. Cost with additive is 9 cents. Total base EC @ full strength appx 1.5!:))
(MG only represents appx 33% of the total weight of base salts)

A small additional note to why I'm still using MG and better-gro; is that they do NOT have additional Ca and Mg nor as high of traces. This allows the full profile to end up at a lower EC than would occur with other mfg's. There is also a very low cost with these products, but I don't consider that a "factor" per se.

I will try to connect some of this, as I'm seeing it as of now.
-It is well documented that the combination of these 3 forms of nitrate (ammonium, urea and nitrate) can potentially result in the highest level of nitrogen uptake / "assimilation"; versus any of the forms individually or in dual combinations. If memory serves me correctly 25% NH4, 25% NO3 and 50% Urea was found best. I feel that aspect is being utilized here and with this method. (there is a common Ag compound of these 3 forms as well).

-Referring back the earlier points of changing N threshold (strain variable); it appears that the combination of the 3 nitrate forms is "less impacted" by the threshold, than was found in tests with NO3 or NO3 and NH4. ( I.E more total Nitrogen without toxicity.) However, it is being applied at "normal" N levels that many may run during this time currently; so the benefit is more related to higher "assimilation" rather than utilization of the higher tolerance (with most strains).

-Referring to another earlier point with changing node space / hormones at the onset of "peak flower". I feel, because this alteration is being made After the hormonal change; there is a lower impact or less "risk" associated with node spacing, resulting from changing N. I believe, by being able to provide a potentially higher level of nitrogen "assimilation" during this time; the auxin / cytokinin ratio can be altered to a larger degree than would otherwise be possible.

-Studies have also shown that plants "can" potentially uptake elements in a fairly quick time-frame after a nutrient application. Some of these studies related to potentially exhausting the elements directly available (contact) within 2-4 hours. If memory serves me correctly the approximate radius for availability "via contact" with roots was around 4mm. One well known advantage of Urea is the "timed-release" fashion which it's broken down / made available. That along with normal osmotic movement can help to provide a more constant N to roots.

-Lastly, the bonus that urea releases carbonates when becoming available as ammonium helps to prevent lowering of pH and pH fluctuations.

(I cannot lay my hands on the documentation, so I will leave this as 'speculation'; but I'm pretty darn sure Urea helped to increase the availability of Molybdenum too.)

From testing with the few strains that I have atm, there are some positive things that may be seen.
-Has the potential, with custom profiles; to match with most growers current N,P and K ppms. I.e. No change in NPK values are required to see the benefit.
-Potential increase in dry weight, variable on strain and genotype.
-Longer / Taller "Stacking" with no, or extremely minimal, increase in node space.
-More "focused" development towards top and uppermost auxillary branches.
-Continued (slow) vertical development throughout application period.
-Longer period of viability for pistils or development of calyx. (more white hairs longer)
-Energy spent on growing more calyx as opposed to "swelling".
-In some cases, faster leaf maturation of new growth during this period.
-Leaves may get darker green and begin to pray if they were not prior.

From the way this is being done, there are a few possible negatives that may be seen.
- Remember do not water any more than what will be used in a 24hr maximum.
-If you have any symptoms of Mg deficiency, foliar spray.
-If your strain needs more Nitrogen in the formula as given (possible); raise Calcium nitrate.
-If there are any symptoms of ammonium toxicity (upper mature edge necrosis) (have seen tiny bit once); lower your average medium "saturation percentage" (lowering urea availability) (and /or) If still seen, lower ammonium percentage.
-With Calcium. As mentioned in steps above, you want your early flower formula to be high in Ca (as it probably is) and to have a good Ca saturation when starting the Urea Formula. Also, if you have a Calcium hungry strain or have any worries of Calcium; Foliar apply Ca as a preventative. If you still note a Ca deficiency you can; preferably allow the EC to raise slightly in Early flower along with foliar. If you Still have an issue, start the Urea formula a "few' days later.
**Note I have not yet seen a Ca deficiency in anything without using foliar; though I do let the EC raise slightly in early flower to build Ca up.

Negatives revolving around timing:
-Do NOT begin applying this if you are not right at the point of "floral meristem" / begining of "peak flower". Late / or early by more than a few days can impact results.
-If your "stacking" has any real notable amount of overall increase in the Spacing of nodes themselves at the apical cola; you may need to start the formula a few days later.
-If a large portion of the "stacking" is of normal node space and just the top section get's an increase in node spacing; you probably need to start several days earlier.
-If you have a strain prone to foxtailing or note unwanted foxtailing; first lower the N in your formula After the Urea period. If the issue persists, stop the urea formula a few days earlier.

I cannot say for 100% sure since it's variable on strain and genotype, but I wouldn't have brought all this up if I thought the benefit was a rarity. Compared to Nitrate based hydro mix @ the same npk. My Bullrider responds stupid well and most notably of all I've tested (this trait has to occur again somewhere). One Pheno of headband (almost done) I "believe" has marginal to little positive(jury still out); though had no negative. However another pheno of headband responded "mostly" as was expected. my GTH#1 and Cannatonic responded well also. Both Pheno's of Fuel (one for sure) seem to be responding well too. Overall I would make minor adjustments with each strain if I were to have it as a main, but outside of that; I'm happy as a clam.

- In organic potted soil. Not sure how it could be used other than adding some urea directly at the right time.
- In Active or passive hydroponic setups with aeration / circulating reservoirs.
- If you have a hybrid that is already at the "theoretical" maximum yield and has perfect form / structure / density etc. I would not expect to see much change. (i.e. probably aready best suited for No3 with minimal change to N threshold going into flower)