This is not true.
Yes, they DO put a "bell" on the penis head and yes, they DO tie a string around the foreskin and yes it DOES essentially rot the foreskin off.
However, in order to get the plastibell mechanism onto the head of the penis, the foreskin still has to be ripped away from the head of the penis (the glans) and it still has to be clamped and cut in order to fit the plastibell onto it. The only difference between these methods is you don't cut the foreskin completely off like with the gomco clamp. It's cut part of the way.
If you don't believe me, feel free to look it up.