Alright I’m new to this, but my dad has been doing Crossfit for 3 years now and can do single under then double under perfectly fine. It is when he starts to try to link the double unders when his left wrist will not keep up with his right wrist in terms of speed. He can only manage 2 or 3 before getting whiplash. He’s at the point where he is so frustrated he wants to just quit Crossfit and go to the normal gym. He’s tryed everything online. Any exercises or tips to help his wrist speed get stronger?