Impulse packs are a small device plugged into the impulse engine, with insulated lead points. When a ship has been through trauma it is best to check the points of the packs because if they are decayed to the lead points, when activated can take out the entire impulse deck. A Constitution-class starship usually has a pack for both the port and starboard sides.
After encountering the galactic barrier Lee Kelso checked out the impulse engines and forgot to notice that the points were decayed to the lead. (TOS: "Where No Man Has Gone Before")