If I'm thinking correctly, the Cohen Curl is the one where you're leaning against a wall, somewhat squatting and have your elbows on the inside of your knees while doing a bicep curl? If I'm wrong then disregard this
This exercise I believe targets the bicep directly without involving too much of the shoulders and and such. When you lean your elbow against your leg, same as with a seated bicep curl where you spread your legs and rest your elbow on the inside of your leg, the exercise aims at only, or almost only, targeting your bicep. When you do standing bicep curls, your back and shoulders are also involved, as well as a few other muscles in your arm.
Anyway, I'm no expert, but that's what I've gathered from one or two weight training classes that I've taken and a book that I've read. I may be WAY off here, so don't take anything I say too much to heart. Hopefully someone else can give you a more sure answer. Cheers!