When Sefton gets to reveal who the mole really is.
After which Price tries to make a run for it through the trap door. The men quickly pull him out to which he tries to shout for the guards to help. Everyone (save Johnny of course) dogpiles on him and holds him down while the searchlight pass over unaware.
The climax: The prisoners know that the guards are waiting for either the escape attempt or for the mole to alert them, so they combine the two. While the escape attempt is kicking off on one side of the camp, the mole is thrown out of the hut with cans tied to his feet in order to attract attention. While the guards are busy shooting their own spy to death, the Americans have gone under the fence and are on their way to freedom.
Further awesome: Sefton was worried about how to deal with a mole because the Germans could just kill the lot of them or ship the mole to another station. By making it look like an escape attempt when he was shot, they can't retaliate as they were the killers.