Continuously firing or charging tears for 3 seconds causes Isaac to drop a Butt Bomb. Only 1 Butt Bomb can be dropped per charging cycle, so you'll need to stop firing and then charge again to drop another Butt Bomb. Taking Damage will interrupt the cycle.
Butt Bombs explode for 60 damage like normal bombs, but also deal 10 damage to all enemies in the room and daze them for a short period of time.
Essentially grants unlimited bombs, as you can just reset the charge time over and over to bomb tinted rocks and to get into Secret and Super Secret Rooms.