If the monsters are closer to the decoy than Isaac, they will attack the decoy.
Homing shots from enemies will always be attracted to the decoy if they are closer to it than Isaac.
Punching Bag will also block enemy shots.
Punching Bag will die after taking too much damage and eventually respawn after a long time. Exiting and continuing the run will cause it to reappear right away.
Punching Bag will reappear instantly each time a familiar or orbital item is picked up.
Notes
The Punching Bag will be targeted by enemies if it is closer to them than Isaac, for up to 6 seconds. Then they will target Isaac for 6 seconds regardless of distance, before acknowledging the Punching Bag again.
The Punching Bag will die instantly by enemies' beam-type attacks, such as Brimstone or by Uriel's and Gabriel's beam. This also includes enemies' Technology lasers such as Eye and Camillo Jr.
Interactions
Box of Friends: Adds another Punching Bag. This may cause enemies to become unpredictable, so use with caution.
If the Punching Bag has died, it can be forced to respawn with a double upon using the Box of Friends.
This item is used in The Guardian (challenge #30).
A punching bag, used in this context, is a target of frequent criticism and abuse.
The term "scape goat" in the pickup quote means someone who is blamed for problems that were caused by another person, or being treated like a punching bag.
Mulligan familiar that walks around the room and acts as a decoy for monsters.
Homing shots from enemies will always be attracted to the decoy if they are closer to it than Isaac.
Punching Bag will also block enemy shots.
Punching Bag will die after taking too much damage and eventually respawn after a long time. Exiting and continuing the run will cause it to reappear right away.
Brimstone or by 
Uriel's and 
Gabriel's beam. This also includes enemies' 
Technology lasers such as 
Eye and 
Camillo Jr.
Box of Friends: Adds another Punching Bag. This may cause enemies to become unpredictable, so use with caution.
The Binding of Isaac: Rebirth
