The Legal Trap is a trap that ocurs in the Philidor Defence. It ocurs when Black tries to pin a knight to the queen.
Black is down a pawn with no compensation.
Sometimes, h3 is not necessary.
This example shows an example of the abiliity to ommit h3. Also shown is what happens if h3 should be played, but White just rushes into the trap.
The best thing about the legal trap is that even if black does not fall into it, White still has a lot of momentum. If black doesn't even try to pin the knight, white will still have a good game.
This is important because otherwise, White would have a hard time if the trap fails.