The electro-sapper is the default secondary weapon for the spy capable of powering down and destroying enemy buildings. The Electro Sapper can be destroyed if the engineer hits the building 2 times with their wrench. (the pyro only needs to hit it once with a melee weapon that can remove sappers) Upon applying the sapper the Engineer will be notified and will say "Spy sappin my (building)"
As the Sapper decreases health you can lower the time it takes to destroy a building by shooting it with your revolver. The sapper alone can take down a level 3 sentry in about 6 seconds it should be noted that putting a sapper on a teleporter both of the teleporters will be effected, however an engineer can destroy both sappers with by hitting only one of them.