A subspace tractor beam was a technology initially conceived of by B'Elanna Torres in 2371, aboard the USS Voyager, when attempting to rescue what appeared to be a vessel trapped within the event horizon of a type-4 quantum singularity.
Since the subspace interference was too high to use a conventional tractor beam, Torres came up with the idea of remodulating the beam to match the interference. To do so took approximately three hours, and required routing more power to the emitter array. The plan initially worked, and the beam was able to penetrate the event horizon, but the new relays that the crew had installed on the power grid were unable to handle the stress, causing the emitter relays to lock. This necessitated Lieutenant Joe Carey physically cutting the main power feed in order to disengage the tractor beam. (VOY: "Parallax")