If you'll watch Doc Watson, one of the best flat pickers of all times, his arm is stiff with hardly any wrist movement. He's been doing it for more than 60 years now. I'm not going to be the one to tell hin he's doing it wrong. This is probably not good advice, I know a lot will disagree with me. But I go with the "Do what feels best" theory. ( and that's almost all I know about theory)