The balance wheel, which is the heaviest wheel in a watch, runs on a staff (axle) with very small pivots (tips), around 1/10th of a mm in diameter, that spin back and forth in jewels, lubricated by a tiny captive bubble of oil. If the watch is dropped, these pivots typically break off, often also cracking or shattering the jewel they run in, and the watch stops. This image shows a pivot that has been distorted but not quite broken off. This watch would still tick a little bit, but clearly a new staff is needed.
This shows the old broken staff and its replacement with good straight pivots. The new staff is riveted into place, the roller and hairspring are pressed back onto the balance assembly, broken jewels replaced, and after a full service, the watch runs again.