Well, firstly, one shouldn't count on being able to just grab a bodypart and then
rip it off, since that would require super-toribashian strength. It is possibly to do
it in a few rare cases, but don't count on it in general.
Instead, one should strike Uke with a lot of force, preferrably directly on the joint.
It may seem a bit hard to do at first, but then you can just use grabby hands
to make your hands hit with the maximum amount of energy.
Also, use many joints. If you are to do a punch from the side, contracting your
left pec and your left elbow isn't going to give a good blow. Instead, one should
like, contract the other pec as well, rotate the chest with the blow etc.
Practice makes perfect! :P