American Football Rules IFAF Approved Ruling 3-5-3:IV

IV. At the end of third down, Team B sends in its kick-return team. The responsible officials count the Team B players and are positive that Team B has 12 players on the field of play. After approximately four seconds, the officials sound their whistles and drop their penalty
markers. RULING: Dead-ball foul, substitution violation. Penalty — Five yards from the succeeding spot. [Cited by 3-5-3-b]