An outside passed pawn is a passed pawn far from the other pawns. Even with equal material it often wins: push it, and the enemy king has to go and stop it. A king can't be on both sides of the board, so while it chases your pawn, your king walks over and takes its pawns.
Every move is checked with an endgame tablebase, which knows the exact result of every position with up to seven pieces. The computer defends perfectly, and if a move throws the result away, the trainer stops and shows you a better one.
