Constant cable tension that cooks your forearm flexors.
Keep your forearms pinned to the bench, the motion is only at the wrists.
Wrist curls hit the forearm flexors directly, and doing them off a low cable fixes the free-weight version of the biggest flaw, tension vanishing at the bottom. The stack keeps pulling through the entire range, including the moment the bar rolls down to your fingertips, which is where most of the growth stimulus lives. Grip strength carries over to every pull in your programme, so this earns its few sets at the end of an arm or back day.
The movement is the same but the resistance curve is smoother. A cable keeps pulling when the bar is down at your fingertips, exactly where a free weight goes light, so you get a loaded stretch on every rep.
Yes, that is the money part of the rep. Letting your hands open slightly at the bottom stretches the forearm flexors under load, just do it slowly rather than dumping the bar into your fingertips.
They recover fast, so two or three sessions a week is fine. Keep reps around 10 to 20 and chase the stretch and the squeeze rather than the biggest pin on the stack.
