A magnetic constrictor in use aboard the USS Voyager.
Magnetic constrictors are specialized variants of magnetic conduits that are used on warp cores, impulse thrusters and deflector dishes of Federation starships. They shrink to apply huge pressure to particles caught within the field, this allows particles to be fired out of small holes or weak points in the containment field. In a starship's warp core anti-matter particles are fired into the matter-antimatter reaction assembly using a magnetic constrictor.
In 2063, the crew of the USS Enterprise-E learned that the Borg were attempting to transform the ship's deflector dish into an interplexing beacon to signal the Borg from the 21st century for reinforcements. Thus, Captain Picard ordered the separation of the secondary dish by disengaging the magnetic constrictors. (Star Trek: First Contact)
In 2371, when the USS Voyager was pulled into the Delta Quadrant by the Caretaker, Captain Janeway ordered that the magnetic constrictors be shut down to avert a warp core breach. (VOY: "Caretaker")
In 2372, B'Elanna Torres destroyed the self-guided missile Dreadnought by burning through the magnetic constrictor casing of its warp core with her phaser. (VOY: "Dreadnought")
In 2373, Chief O'Brien claimed he wanted to test new magnetic constrictors on a runabout's impulse thrusters when he took this runabout to the Bajoran wormhole. (DS9: "The Assignment")