A white glass cannon.
Upon activation, Isaac will raise the cannon above his head, lowering his movement speed. On pressing a fire button, Isaac will fire a enormous tear in the chosen direction. Additionally, Isaac's red hearts are set to half a red heart. Soul hearts and black hearts are not affected.
Playing as ??? or Azazel, soul hearts and black hearts are affected.
A "glass cannon" is a common term in RPG games for classes that deal a lot of damage, but have low health.