A reversing mechanism for a pneumatically or hydraulically powered tool having a rotor adapted to rotate in either of first and second rotational directions. The reversing mechanism allows a user to actuate a button and rotate a valve to direct air flow through the tool. By pressing the button, the button will move a base laterally, and in doing so, rotates the valve. Rotating the valve then aligns a barrier of the valve in a direction tangential to the selected rotational direction of the tool, better directing forced air or fluid and more efficiently distributing the air or fluid in the selected rotational direction.