Actually, because of your lack of light, you want to make your canopy as even as possible and force those lateral branches up into the canopy. Pinching is easy, put on some sterile gloves, or wash your hands very well, firmly grab the top of the plants stem just below the apical meristem (just below the newest growth, where the stem is thick) or below the top node, either way --- then between your thump and forefinger gently squeeze the stem until you hear a soft crack. This damage to the stem will heal quickly, but in the mean time, all those hormones which were going to the growing tip will now be distribted among the lateral branches. If you hit them with light ferts and some kelp, you'll probably notice those lateral stems shoot up in just a few days. The main stem will also be strengthened. Basically, yove' damaged the xylem and phloem, the parts of the stem responsible for transport, and everything below the break will start to bulk up because the nutrients no longer reach the highest growing tip.
When you flip them, you can then trim all the lower shoots that reduce canopy penetration.
The best part? If you pinch too hard and break off the top, you'v topped your plant --- which is still cool. Just take a sterile razor and make the cut more clean and then you've got your first clone as well.
