(image from mindlessmunkey)
To make the perfect jazz hands, you must relax your hands, then extend your fingers. It’s important to leave your palm relaxed.
If you just stretch out your hands, they look too uptight and anxious.
If you just let your hands flop there, they look as though they have nothing to say.
But if you wake up your fingers, keep your palm relaxed, maybe set your fingers together and let your thumb stand up, then lift your arms from the elbow, out to your sides… if you do all that, then you will have excellent jazz hands. And you will be jazz dancing.