Impulse packs are small cylindrical devices that are plugged into a starship's impulse engines, and are insulated with lead points. The impulse packs have to be regularly checked because if a lead point has decayed, it could knock out the entire impulse engine.
Following the USS Enterprise's encounter with the galactic barrier in the year 2265, Lieutenant Lee Kelso checked the ship's impulse packs and failed to locate one with decayed points. When Kelso visited Gary Mitchell in the Enterprise sickbay, Mitchell used his growing god-like abilities to pluck an image of the decayed point from Kelso's mind, and informed him of what he'd missed. (TOS episode & Star Trek 8 novelization: Where No Man Has Gone Before)