IMHO you have to be at least partially erect to a) get it on properly and b) get the most out of a session. I used to use adult entertainment but that's too much hassle on a regular basis. I just jack to about 80%, which is good enough, then just use mental imagery, plus the sensation from the tube as well, to maintain that throughout. I also find that swinging the tube back and forth using pelvic flexion both helps in maintaining the erection and also gives a bit of lig stretch.