Earlier I was too lazy to include in my post I start my extreme uli in 2 steps. First I wrap (sock+leather strap) with an erection and put on a hoseclamp which I tighten a little so it works as cockring. Then I edge for a few moments till I’m really hard again afterwhich I tighten the hoseclamp all the way possible.
My dick doesn’t get much more expanded till I start tightening and increasing the pressure inside. I don’t see a problem with clamping off an erection as it has always worked for me. If I used a cord you would have been right then clamping off in/outflow would not do anything, but with a clamp+wrap you squeeze a larger area and force extra blood inside increasing the pressure causing expansion (this does require kegeling during tightening so the blood is not forced out). You can see the effect in this post:
Maybe this is pretty much what you are doing too but I just wanted to clarify my previous post.