Cluster magnets are fired two-at-a-time onto opposite walls. They affect the polygrain metals found in the Pawn balls, pulling them out of your way. They've also been known to confuse Sentry ball's vision giving you just enough time to sneak a killer barrier shot through. Be sure to fire the barrier in the opposite direction to the magnets or they will be wasted. It may take some practice, but it's worth it -- magnets are very useful in higher levels.