Again at the end of Chapter 5-3 just after "Bad Blood." Wesker will depart and leave you alone with Jill. Move closely to her but be careful, too close and you'll get a boot to the face. The game will prompt you with the button to try and reason with her (if you are Chris); it may take a few of these prompts but Jill will then become confused. Sheva will grab Jill and give you the opertunity to try and grab the chest piece or shoot it, shoot it for now until it begins sparking. Now reason with Jill and when Sheva grabs her press to grab the chest piece then mash to try and rip it out. It will take a few attempts of this to finish the battle.
This will not unlock if you rip out the chest piece when Jill is on the floor.