Abstract: The present invention provides actuation mechanisms that incorporate a transmission assembly that allows the mechanisms to cause actuation of workpieces according to a plurality of transmission modes (e.g., at least one forward transmission mode and at least one reverse transmission mode) on demand. Motion, direction, and/or force can be controlled by selecting the corresponding transmission mode. The mechanisms preferably are trigger-actuated by hand (i.e., manually) to cause movement of a workpiece in a desired direction. Desired directions can be linear or nonlinear. The same hand used for trigger action can also be used to change transmission modes in many modes of practice, even while using substantially the same grip used for trigger actuation. In other instances, actuation can be automated rather than manual. Preferably, both actuation and witching among transmission modes can be accomplished with one hand, even while maintaining substantially the same grip that is used for actuation.