Enemies that touch Isaac will bounce off him and propel in the opposite direction. If they hit an obstacle or wall while bouncing, they will take damage.
If an enemy bounces into another enemy, the second enemy will also take damage.
When an enemy bounces off of Isaac, there's a 50% chance that Isaac won't receive contact damage.
Additionally grants a 50% chance for projectiles to bounce off of Isaac without taking damage.
Notes
The damage enemies receive when they get bounced into a wall or an obstacle is equal to 8 + 2 * floor.
The same damage value applies when an enemy gets bounced into by another enemy.
Enemies that don't deal contact damage cannot be bounced off.
Since contact damage is applied rapidly, it is possible to end up taking damage and then immediately bouncing the enemy off.
Bounced off projectiles deal a flat damage that doesn't scale with the current floor or Isaac's stats. This damage is usually 6, but can slightly vary depending on the enemy that shot the projectile.
Lasers cannot be bounced off.
The bouncing effect also works against Fire Places and spiked enemies such as Poky, Slide, and Grudge, granting a chance to avoid unintentional damage.
Isaac can also bounce off Fly Bombs spawned by Blasters and even deny damage from their explosion.
Gnawed Leaf: The bouncing effect works while Isaac is invincible.
Serpent's Kiss: Allows Isaac to bounce enemies and poison them at the same time, with a decent chance to get a lot more health back in the form of Black Hearts than originally spent.
This item can be considered a direct upgrade to Infamy, as the chances to deny projectile damage are equal, but Belly Jelly additionally works with contact damage, unlike Infamy.
Belly Jelly is just the brand of jelly beans Jelly Belly®, but the words are flipped. Given the game's theme, this also might be a reference to the jelly spread on the belly before an ultrasound.
Fire Places and spiked enemies such as 
Poky, 
Slide, and 
Grudge, granting a chance to avoid unintentional damage.
Fly Bombs spawned by 
Blasters and even deny damage from their explosion.
Bombs and 
Troll Bombs cannot be bounced off.
Tainted Jacob from being turned into his Lost form by Dark Esau.
Aries: Retains the chance to bounce enemies while preventing damage simultaneously.
Book of Shadows / 
Unicorn Stump: Can be used as a powerful offensive item since Isaac can still bounce off enemies while invincible.
Cone Head / 
Host Hat / 
Infamy: Offers an additional chance to negate damage.
Gnawed Leaf: The bouncing effect works while Isaac is invincible.
Serpent's Kiss: Allows Isaac to bounce enemies and poison them at the same time, with a decent chance to get a lot more health back in the form of 
Black Hearts than originally spent.
Spindown Dice: Belly Jelly rerolls into 
Glitched Crown.
Panic Button: The active item will still be activated if Belly Jelly blocks damage.
Bucket of Lard and 
Binge Eater.
Tainted Magdalene Unlocks 
The Beast Unlocks 
The Binding of Isaac: Rebirth
