A force activator was a component of the tractor beam for Federation starships. The activator was controlled from the engine room. The beam generated could be used for pulling in or repulsing an object.
In 2364, Wesley Crusher diverted power from the warp engines into the force activator, then rerouting the power back into the engines, a repulsor beam was generated. The beam pushed the USS Enterprise-D away from the SS Tsiolkovsky and the approaching stellar core fragment. This gave Lieutenant Commander Data the minutes needed for restoring the command computer. (TNG: "The Naked Now")