Rule 3 – Section 5 – Article 2 – Legal Substitutions
e. When Team A sends in its substitutes, the officials will not allow the ball to be snapped until Team B has been given an opportunity to substitute. While in the process of substitution or simulated substitution, Team A is prohibited from rushing quickly to the line of scrimmage with the obvious attempt of creating a defensive disadvantage. If the ball is ready for play, the game officials will not permit the ball to be snapped until Team B has placed substitutes in position and replaced players have left the field of play. Team B must react promptly with its substitutes.