A drive unit that is movable between an unfired and a fired position. The drive unit comprises a handle member, a first drive gear having a first axis of rotation, and a second drive gear, engaged to the first drive gear at a mesh point, and having a second axis of rotation. The mesh point moves with respect to the axes of rotation as the handle member moves from the unfired position to the fired position. The drive unit drives an implement only when it is moved from the unfired position to the fired position.