If your shoulder is getting sore, you are pushing down to hard. You should only anchor it in your shoulder. In the area to the inside of the shoulder bone. Some make a longer handle and lay it on top of the shoulder. You have to anchor it there only. If you watch when I teach in in the NYC CNC show at I think it's at 15 min in I tell the student to bend over forward feeling like your going to fall forward and before striking the flaker practice leaning backwards and drag it in a straight line. The handle is rolled to the left of the blade where I have the black line. You squeeze the handle and as you strike the opposite side of thee handle, the tight grip springs back to the spot you had it before you hit it.
It is a hand ,eye, body co-ordination. You do not rotate your hand, you hold it solid and the handle rotates back and forth from spring action. Practice dragging it backwards a lot before hitting it. After each line is pulled across the plate shift your feet in a move your toes to the left or right depending it your moving in that direction, pivot on the balls of you feet a couple of inches and then your heels, toes -, heels shift. and pick up the handle at the back of the pull back and lift the handle up aand set it over a 1/2" at the top and drag it back and after you get the hang of that start to hit the handle. with a hammer, straight at your belly, with your hand, hit it on the up swing as you rotate your hand in a circle hit. It is a hell of a lot easier to show it, then write it...lol