clamping is when you stroke your cock into an erect state, then use a cock ring or whatever over your erect cock to hold the blood in. keep pumping or stroking, letting addl blood flow into your penis, and then hold it with the ring. I would do this and then push the blood to the end of my penis. a good 15 minutes of this, and your head is visibly bigger. it also increases girth, along the shaft, and in my case right below the head. results are temporary at first, but will become permanent over time.
be careful, don't leave the clamp on too long, or at least let the blood circulate. also some red spots will appear on your penis head if you get too aggressive.
I basically use a thick rubbber band for a clamp. my head was kinda of small, long and narrow, sitting on a thick cock. now it is full and round, almost like a ball overfilled with air. the old head was better for penetration, but my wife likes the bigger head once it gets up in her.