When your opponent declares an attack while you control a monster: Target 1 face-up monster your opponent controls, except the attacking monster; take control of that target until the end of the Battle Phase, and if you do, it is attacked instead, and you proceed to damage calculation.
it doesnt work a lot of the times and i have no idea why, it seems the ruling is pretty messed up on this one.
The standard situation where this always work should be.
You have 1 Monster, enemy has 2 Monsters.
he attacks, you use it,
Sadly you often can't hope they fix the ruling on this one, for me just not usable the way it is.