Big Girtha always said that it was ok to clamp, and then let yourself go flaccid while clamped. I never believed it could work that way. He used to site the fact that after he was flaccid in the clamp, his girth would still be expanded beyond his erect girth, and he would measure to verify it.
Now that I think about it, maybe he was on to something. Allowing the erection to subside relaxes the tunica in the longitudinal direction, and maybe allows more expansion in the circumferential direction. It never made sense to me because it doesn’t feel like anything is being stretched, but maybe the expansion itself stimulates some growth or remodeling toward the larger girth.