The Rise Up Fist

This detail-rich fist rewards its users with a narrow wrist and twisting forearm.

Once you get past the full 3.5 inch wide knuckles your ass can close around the narrow wrist. Once there, you'll be able to expand your ass again as you sink down the gradually expanding forearm. The forearm even features a little twist, so you'll be able to enjoy turning it inside of you. All in all it's a good tool for fisting training.