There is a peak point where the plant starts to use the majority of it's conserved P&K (N as well, however we are fine with depletion of Nitrogen at this point). We want to focus on overall health and flowering production with the P&K.
About mid-way through the plants flowering cycle it will hit a spike, where you can add 50% more to your original ratios of P&K.
Quick Edit - I am not sure if week 4-5 for a PK spike on a 12 week strain would be as beneficial as a spike around week 5-7.... has anyone tested PK spikes at early weeks with 10-12 week strains, or is it always mid-point?
It has worked well for me. I have tried with a straight regimen as well as a staggered regimen and found PK spiking does show significant results IMO. (8-9 gram tops compared to 4-5 gram)