foot moves and you have a few options:

throw to an occupied base
fake a throw to 2nd base that is occupied
throw to an unoccupied base where a runner is attempting to steal
Throw home in one continuous movement.

Failure to do any of those 4 things is a balk.

Also, gotta come set. Can't start and stop. Can't fake to a base other than 2nd (NFHS allows any base but 1st).