What is the 'Enemy Control' glitch?

There is a FAQ on it in the FAQ section.

Basically what you do is have any unit attacked by an enemy standing on a fire panel or a panel where a tree that you can knock down to make a bridge once stood. It doesn't matter how the battle turns out, but the enemy must survive. Now just before the enemy that attacked grays out to show that that unit has been moved, reset your game, and then continue it and you should be in full control of the enemy units until you end the turn. It's recommended that you use a unit who cannot attack back (remove all weapons) so you can do it longer, or use a Summoner's Phantom if you don't want to risk your characters.

While you control them, you can make them do anything that you would normally be able to do with your own units. You can't trade the enemy's items with the blue units, but you can give all of the enemy's items to your main lord that has the Supply.