What counts as a false start?

Under World Athletics Rule TR16.7, once an athlete has taken their full and final starting position, they must not commence their start until after the report of the gun. If, in the starter's judgement, they do, it is a false start. Commencement is defined as:

• Crouch start: any motion that includes or results in one or both feet losing contact with the foot plates of the blocks, or one or both hands losing contact with the ground.
• Standing start (races over 400m, except some relays): any motion that results in one or both feet losing contact with the ground.

It is also a false start if an athlete began a movement before the gun that wasn't stopped and carried on into the start (a "rolling start").

Not a false start: other movements that don't break contact, such as lifting and lowering the hips. These may instead be dealt with as improper conduct. In standing starts, an accidental over-balance should be treated as "unsteady" rather than a false start, and an athlete pushed or jostled over the line isn't penalised.

When a certified start information system is used, its evidence is a resource for the officials in making the decision.