It makes sense to me, maybe for 12-48 hours. It's giving the plant an extended period of growth. You could do it at any stage of flower; perhaps folks do it near the chop because extended dark may stress the plant. If you're going to do it, I'd suggest giving the plants as much light as you possibly can without burning them during the last 12 hours of light; this way they've stored up an unusual amount of energy to be expended during the dark cycle. I've used this technique before, years ago; I honestly have no record of the results. However, what I do now, is flower under MH light during the last 2-3 weeks in order to force the plant to produce more resin (as a "sunblock" against the UV rays from the MH HID light). I also flower under MH for the first 2-3 weeks to ease the transition from Veg to Flower. ::breath:: GL.