Sit Tight: Use at the start of a round. You do not activate any groups this round until you have more ready deployment cards than your opponent.
What happens if two players with an equal number of deployments both play this card? As written, neither of them would ever be able to activate a group. This would either end the game at time, or cause a stalemate.
This is an extremely rare case, but it could use an FAQ or ruling for clarity sake.