Definately do them at all erection levels. Some people think that there is some length gain when jelqing at lower or near flaccid levels, but I don't know if that is a certainty. When more flaccid you are stretching your shaft more then when erect. When I am more flaccid and do these UJ's I am moving a mass of blood up my shaft, and I can actually feel that mass. I don't feel that when jelqing at higher erection levels. However, my glans is nearly rock hard regardless of how erect I am, so in all cases I am moving large amounts of blood into it. If I touch my glans when fully erect but am not jelqing, my glans is soft, some people here apparently have hard glans when erect. Since mine is soft, the only reason it is hard when jelqing is because of the blood I have forced into it. When 70% or more erect, the UJ's will affect vein prominence. Just like a true horse squeeze, these mini squeezes will bulge your unit and blood veins. Some people don't like a lot of vein, but I personally do. I suggest that you first get your normal grip, then apply your middle finger (don't wrap both fingers around at the same time). When that second finger gets into place, you will immediately feel the difference (not quite as much if more flaccid).
I am sure others have been doing this exercise (Kingpole for one), just didn't post anything. But when I started doing them I fell in love with them immediately, and wanted to share my thoughts with others. Kingpole mentioned using more than two fingers, but the reality for the other 99.9% of us is that our shafts aren't long enough for that to be effective. I think these UJ's, when done by someone who is conditioned, absolutely blow normal jelqs out of the water for effectiveness. Just before you repeat step 2, throw in a kegel to take it up even one more notch.