It can’t be a broomstick with a string and a 5 pound plate. That’s the equivalent of the pink dumbbells!
The roller has to be thick, at least axle thickness, to work your thumbs. The roller has to be supported, because your shoulders can’t support enough weight to challenge your wrists and hands. A string won’t work because you need some poundage on that thing!
Lots of guys aren’t into it. But Magnus Samuelsson used one, and he was pretty good.
If you’ve ever felt something like an axle or thick dumbbell “roll” against your grip, the wrist roller is pretty specific training for that.
It’s a good “general” grip exercise. Levering sledge hammers, then doing wrist curls and then doing rubber band finger extensor exercises is boring and takes all day. A couple rolls go fast, and work a bunch of that stuff.
If you roll knuckles back, you work the forearm extensors and stretch the forearm flexors with every twist. This is great therapy/rehab if you have wrist or elbow issues.