A hand-held sports or exercise training device configured to improve the swing of a user that includes a hollow body defining a interior volume extending within the device and a slidable mass having an passage that extends through the mass in the direction of movement, or, alternative, one or more passages that allow air flow are contained in a sleeve housing the slidable mass and/or in the bat end. The slidable mass is contained within the interior volume or an added sleeve and is in contact with an interior circumference of the hollow body or sleeve. As a user swings the device, the moment of inertia for the device changes such that swing mechanics and/or timing can be improved.