Having this item will spawn a friendly Boom Fly that explodes and heavily damages enemies that get caught in the blast. Be aware as the explosion can still harm Isaac unless he has Pyromaniac or some other form of damage negation. A new Boom Fly spawns a short time after the last one detonates or when moving between rooms. When the BBF spawnsin a new room, it appears at Isaac's coordinates and instantly begins travelling in one of the four cardinal 45 degree directions. When the BBF spawns after a previous explosion, it appears in a random location and begins moving in the same way.
The BBF is incredibly effective against large enemies, particularly Larry Jr, who can be killed in two collisions with the BBF and often catches it along its side.
The BBF's explosion can be used to blast open doors if an enemy is lured near the door and hit.