Maybe Avocet8 will chime in and give you some solid advise on this, I still consider myself a novice here, although I have made some pretty good pumping gains. I really don’t think it matters about the position as long as you keep a good seal. Secondly, I always watch porn in the beginning of a set, going into the tube, because it seems there is less soreness after a session when I’ve entered the cylinder hard, but once inside and a good seal is achieved I turn from the porn to read T’s Place or go about the house doing nonsexual multitasking. Even though my natural erection fades the pressure keeps the penis erect enough for the pump to do its controlled damage. The dangerous thing about staying stimulated all during the sessions is, because pumping unlike hanging, actually feels good. If you spend the entire 20-minute set watching porno, fucking the tube, you become tempted to either ejaculate in the cylinder, or worse, stay longer than you should.
Just my two cents
Big