A sport training device wearable by a human user, includes an oblong resilient, flexible member diagonally connecting one side of a shoulder harness to the opposite side of a separate hip harness. The member is tensioned in the user's upright, untwisted standing position. During a swing or a baseball-type pitching action, as the user's hips properly rotate with respect to the user's shoulders, the tension reduces thus encouraging and indicating to the user the preferred relative hip rotation action. Adjustments are provided adjust the length and tension of the resilient member. A member which deflects uniformly under a compressive load can be used as a measuring device of the relative angle between the hip and shoulder.