Once thrown, the Friendly Ball will instantly capture any non-boss enemy it hits, removing them from the room.
Walking over a Friendly Ball after it captures an enemy will instantly recharge the item.
The next time the Friendly Ball is thrown, it will release the previously captured enemy as a friendly enemy.
Released enemies persist between rooms and floors and will follow Isaac until they die.
Friendly enemies cannot harm Isaac and do not need to be defeated to clear a room.
If Isaac leaves the room before the monster is released, the monster will stay in the ball until it is thrown again.
If the Friendly Ball does not capture an enemy, it will simply disappear after a couple seconds being on the ground. The next throw will work the same.
All enemies that aren't invincible and are not designated as a boss or mini-boss can be captured by the Friendly Ball. This includes enemies summoned from bosses such as Brownie's spawned Dips or The Cage's unique Vises.
One exception to this is if you capture a Portal, as it can spawn enemies such as Death's Head.
It is generally wasteful to use this item on Mulligans and their variants as they will immediately rush towards enemies and try to kill themselves, leaving behind friendly debris.
If the captured enemy is a Globin variant (besides Black Globins for their lack of regenerative abilities) or a Dark Red Champion, the enemy will regenerate after dying, granting a much longer lifespan.
Wall Creeps and their variants stick to walls, backing far away from any potential danger they might encounter.
Stationary enemies that move underground (such as Round Worms or Lumps) work surprisingly well as an ally as they shoot foes from far away and rarely ever get hurt.
Mom's Hand and Mom's Dead Hand will both one-shot almost all normal enemies and greatly damage most bosses with an incredibly small window for the hands to be hit, making them ideal for capturing.
Brimstone-capable enemies are very potent as their lasers can cause an obscene amount of damage to both enemies and most bosses.
If the captured enemy is one that spawns enemies, those enemies will be friendly as well.
If the ally is immune to damage (such as a Walking Host while its head is lowered), it will not take contact damage.
If the captured enemy is a Solid White Champion, it will be completely invincible, as it cannot be killed until the room counts as being cleared, at which point it can be killed by spikes or a Wall Hugger.
If the captured enemy is a Pulsing Red Champion, it will regenerate the health of other monsters in the room, including friendly ones.
If an enemy that leaves behind a trail of damaging creep is caught, the creep created by the friendly mob will still damage Isaac when he walks over it.
All creep created by a friendly enemy will only hurt grounded enemies and not the player.
The ball will only capture one segment of a Grub, one Buttlicker, or one Swarm Fly. However, when releasing a Swarm Fly, a new swarm is generated for it.
If a Heart is caught and then released, a Mask can use the friendly heart to live even if its original heart is destroyed. The room will not be cleared unless the Mask is killed.
The mask will ignore the friendly heart yet can still damage it. Bait the mask to charge towards the heart to destroy both.
If an Ipecac-capable enemy is caught, the explosive Ipecac shots it fires will not hurt Isaac.
This explosion immunity also applies to enemies that explode on death, and even the Troll Bombs left behind by Mulligoons.
The head of Rag Man can be captured by the ball while detached. It will not become a Ragling after being released, and only chases enemies by rolling.
Captured Bishops will protect Isaac with their shield generation ability, as well as all other friendly enemies. Isaac cannot take any damage while the Bishop is alive, as it will protect him with the shield. Protecting the Bishop can be a powerful defensive tool, especially when playing as The Lost.
Captured Cultists will revive killed enemies as friendlies, allowing one to slowly amass an army of friendlies by protecting the Cultist.
Bosses and degraded bosses can be revived by a friendly Cultist, but will not persist between rooms.
The Battery: Walking over a captured enemy will double charge the ball, allowing the enemy to be released, and allowing to catch a new one in the same room, and the process repeats. This allows the Friendly Ball to be used much, much more often, and gathering many more friends. Note: trying to catch a new enemy while the old one is in the process of spawning will result in the ball phasing thru, catching nothing, thus wasting charge.