You sac 25% of your HP and then a little doll comes up to take damage for you until it dies. So as long as the sub is there, you can't take damage unless its a move that hits twice. You also can't be hit by status like Toxic or Will-O-Wisp as long as the sub is up. But if a damaging move would do 25% to the sub, the sub breaks.

It's basically a meat shield for 25% of HP which is good to set up when you force a switch by threatening a KO, like switching in Smogonbird against Breloom.