Hand interfaces (11), (111), (188), (201a), (201b), (202a), (202b), (204), (211), (311), (411), (511), (611), (711), (811), and (911), which improve the performance of many items including: racquets for tennis and other sports; rotary hand tools such as screwdrivers; hockey sticks; and kayak oars. Relative rotation between one of the hand interfaces and a corresponding implement is selectively stopped, and torque is selectively transmitted between the hand interface and the implement, when a user squeezes the implement's
handle. The hand interface can be rotated in either direction on the implement, and relative movement stopped at an
infinite number of positions. The hand interface amplifies the opposing effects of either a firm grasp or a relaxed grasp. Tactile indicator systems (36) with (38), and (236a,b) with (238a,b), glove (204), as well as
handle grips (611), (711), (811), and (911) which rotate on noncircular shafts, enable users to accurately determine rotational positions of the implement to the hand interface. For additional ease of use, adjustable end-stop systems (273a,b) with (275a,b), or (73f,b) with (86f,b) limit the range of rotation between the hand interface and the implement