Sometimes, you'll encounter enemies or NPCs wearing Power Armors, especially raiders. If you kill them with the power armor on, you'll be able to salvage the parts but not the frame itself. If you want to get the free frame, you have to force the user out of the power armor. So how do we do this?
Simply by shooting the fusion core at the back
Now the only way to do this is when your target is not focused on you. Like the screenshot above, if you have good long-range accuracy, you can destroy the fusion core while the enemy is unaware and while crouching, causing you to deal bonus Sneak attack damage. If you have the Penetrator perk, it's possible to reach the fusion core even if the enemy is facing you.
Once destroyed, the damaged fusion core will be ejected from the Power armor, causing a devastating explosion and will cause it to stop moving. This will force the user out and will prevent anyone from using the power armor again unless a new Fusion Core is installed.