Impulse engine relays, also called impulse relays or relay stations, were primary components of the impulse engines aboard a starship.
When Enterprise NX-01 experienced a power surge in the impulse relays in 2151, he noted that instance as "a good example of how we can reroute the energy flow to protect our critical systems." (ENT: "Cold Front")
In 2153, while visiting his family's freighter the ECS Horizon, Travis Mayweather took it upon himself to reroute the impulse relays in Module 5 in order to tie the ship's plasma turrets directly into the impulse engines. (ENT: "Horizon")
When the USS Enterprise was caught in a mysterious force field in orbit of Pyris VII in 2267, Assistant chief engineer LaSalle attempted to counter it by having Engineering divert all power systems to the outer hull. Included in this request was for them to prepare the impulse engines for generation of maximum heat directed to create an electrical field to put a dent in the force field. As it became apparent that their efforts were succeeding, DeSalle further had them channel the entire output of impulse reactors 1-3 into the relay station, to reinforce their efforts. (TOS: "Catspaw")
As the USS Enterprise-D was caught in the tractor beam of a Dyson sphere discovered in 2369, Data noted that the beam's resonance frequency was incompatible with the Enterprise's power systems, and as such, both the warp and impulse engine relays had been overloaded. (TNG: "Relics")