To learn to be relaxed and solid yet free with your hits, try to walk on your fists and feet with belly up. It will both give you endurance and free your shoulders and neck if you pay them attention and will teach you to hit well since you need to do this quietly. No hammering of fists into the floor. Just place them jently with the mass and move on. Don’t try to keep the way your fingers are and relax the hand while it’s closed. Oh and don’t do it on the heels. Relax the foot too.