Why isn't Armature>Separate Bones working for me I'm a blender noob.

I obtained a blender model which has two rigged low-poly hands attached to an armature with shoulders and arms.

I'd like to edit it so that it only has the right forearm and hand.

I think that I should be able to do this by selected the relevant bones and then using ctrl-alt-p to make the arm a separate armature.

But no matter what I try that command doesn't seem to do anything.

I'm using Blender 2.79, on a Mac.
In edit mode, select the bones you want in a different armature, then press P.