taras0000

It's in the UCI rules.

Race stoppages
3.2.048 The race may be stopped only:

1. in the case of fall. If the fall be intentionally caused by a competitor, that competitor shall be relegated or disqualified from the tournament according to the gravity of the fault committed and the other competitor declared the winner. In three or four-up heats, the race shall be immediately restarted with the remaining two or three riders. Should the fall have been caused by a competitor riding too slowly in a curve or by any other unintentional fault, the race shall be restarted and the offending rider shall take the inside of the track.
If the fall is not caused by a competitor committing a fault, commissaires shall decide whether the race is to be restarted with the riders in the same order or whether the positions at the time of the fall should be considered final.
2. in the case of a puncture.
3. in the case of the breakage of an essential part of the bicycle. In all of these three cases, the commissaires shall decide whether the race is to be restarted with the riders in the same order or whether the positions at the time of the incident should be considered final.
4. in the following cases: a) If a rider loses his balance, falls or touches an opponent or the barrier, the race shall be restarted and the rider in question shall take the inside of the track. b) If the starter observes a flagrant infringement and stops the race before the bell rings to indicate the start of the last lap, the commissaires may relegate or disqualify the rider committing the infringement. The other rider shall be declared the winner or in the case of a three or four-up race, it shall be restarted as a two or three-up race. (text modified on 1.01.02) 3.2.049
If the rider committing the infringement is not relegated or disqualified, the race shall be restarted and that rider shall take the inside of the track. (text modified on 1.01.02)