During chapter 8 you are been sent back in time/in a vision of when your mom got killed but this time you can defend her against the alien. During the scene after defeating them when she hugs you you can break apart from her then shoot her a few times. I pressed X to break apart and then shoot her with RT.
***Edit*** thanks to Soulbite to give more precision : The trick is to shoot her three times. Keep holding LT and mash RT until you get three clean shots into your mum.
Can also be done if missed after completing the game. Again thanks to Soulbite for the precision.
I hope i make sense, i am not a native English speaker.