Magnetic constrictors were specialized variants of plasma conduits that were used on warp cores, impulse thrusters and deflector dishes of Federation starships. They shrank to apply huge pressure to particles caught within the field: this allowed particles to be fired out of small holes or weak points in the containment field. In a starship's warp core, antimatter particles were fired into the matter-antimatter reaction assembly using a magnetic constrictor.
In 2371, when the USS Voyager was pulled into the Delta Quadrant by the Caretaker, Captain Janeway ordered that the magnetic constrictors be locked down, in order to avert a warp core breach. (VOY: "Caretaker")
Later that year, Seska instructed Michael Jonas to sabotage Voyager by forcing the magnetic constrictors out of alignment. This caused a plasma overload in the warp core that had to be vented via the warp nacelles, thereby damaging the ship's warp coils and forcing the ship to divert to Hemikek IV for repairs. (VOY: "Lifesigns", "Investigations")
In 2373, Chief O'Brien claimed he wanted to test new magnetic constrictors on a runabout's impulse thrusters, really only intending to pilot that runabout to the mouth of the Bajoran wormhole. (DS9: "The Assignment")