I think it works the same way as F-Iron - as you increase your weight, you get a corresponding increase in strength so that you don't fall and crush your own body. In the case of A-pewter, while you get an increase in strength, you also get the corresponding increase in sensory feedback necessary to not injure yourself with the increased strength. As proprioception involves nerves and receptors that reside in and around muscles, I believe A-pewter enhances the feedback a user gets from them