Cubdriver is correct:

DG: Functions by rigidity in space, Precession is an error.

AI: Functions by rigidity in space, precession is used to correct the horizon to "down". Pendulous vanes (the air valves) allow air flow 90 degrees offset from the needed correction, and precession makes the right correction. If the aircraft spends a significant amount of time in non-level flight, the mechanism will "correct" the horizon to something other than gravity...in this case the precession of the correction mechanism is not helping you.

TC: Precession causes this work.


Another good precesion example: Helicopters. One of the first helicopters was uncontrollable because the controls were rigged logically...ie push forward on the cyclic and the rotor disk tilts forward. Because of precession this caused the force on the disk to be redirected 90 degrees, causing the controls to function 90 degrees off...ie push forward and the helo rolls sideways These early experiements were obviously entertaining to watch...

Functional helicopters all have controls rigged 90 degrees ahead of where the force actually needs to be applied.