If this doesn't stimulate you well, try fixing your lower body around the middle of your thighs (drop your lower body a little away from the support). With a low weight, think of pulling with your back (if you don't grip with your index and middle fingers, your arm strength will be less engaged). While pulling, if you slightly release your arm strength, the weight will pull your back, but only go to the point just before your back loses strength and then pull again. This way, only your back will be engaged continuously. Don't use a heavy weight; try to get a feel for it with a lighter weight.