Backstabs are just melee attacks done while sneaking. However, sometimes I also get a different animation than ususal when murdering someone from behind, although I've only managed to do it once, on a raider I just forced out of a power armor. Is there any way to consistently get a backstab animation? Does it only work on certain enemies? Is getting behind not quite enough?
I'm more than a day into a playthrough with a stealth ninja melee character. I get these animations fairly often.
I'm primarily using General Chao's Revenge and Blitz to melee guys from far away, sometimes behind, sometimes not. Sometimes I sort of chop their heads off, sometimes I impale them through the chest, sometimes I jab the sword straight downward through the top of their ribcage near the collarbone. That last animation takes forever.
I can confirm that they happen in VATS when stealth attacking humans.
I believe I've also got such an animation at least once out of VATS, but I am using VATS almost exclusively. I recall getting these animations regardless of the angle at which I'm facing the enemy, but note that VATS places me behind them when it executes the attack. I don't recall ever getting such an animation on a non-humanoid enemy.
I find these animations annoying, and wish I could turn them off. They delay being able to act again for a while after leaving VATS, which has gotten me killed more than once when there was another person around I didn't see before attacking.
Assuming that you've met the criteria for a backstab animation to be possible, the animation actually occurring seems random. Sometimes it happens, sometimes it doesn't.
It's possible the animations may only trigger with certain weapons. Try a combat knife or a Chinese officer's sword. I've gotten them with both of those for sure.