You’ve seen paired animation before, even if you didn’t know what it was called. Many people have frequently called these “canned animations” or some such. I first discovered the technical name for it is Paired Animation when I saw a production video on Assassin’s Creed Origins, that happens to define what it means.
“We’ve drastically changed the paradigm of what is fight. In previous [Assassin’s Creed games], we used what we call technically a paired animation system. Which means when you swing your weapon, the hero and enemy align, they play an animation together, you wait for it to finish, and then you continue fighting. We went to a hitbox system, which means that anytime you swing your weapon, no matter the distance or if anyone is around you, you’re gonna swing. That means that distance matters. The speed of your weapon, your position relevant to other enemies, [it all] matters. If you have a big spear and are swinging it around you can hit multiple enemies at the same time. It’s not just about the damage anymore, [but about] speed, length, position and the number of enemies you’re fighting.”
In short, it’s when both a player and enemy are involved in the same animation, one of the player attacking, and one of the enemy being hit, neither being allowed to move independently while this is going on. This approach allows animators to have the player manipulate the enemy’s body and limbs in the animation directly. This is necessary for things like catching an opponent’s punch.
So where have you seen Paired animation before? Assassin’s Creed is the obvious one, every game before Origins had paired animations for all combat. The Batman Arkham series uses paired animations for all punches, counters, and takedowns, plus jumping on top of an enemy’s head. Dark souls uses paired animations for backstabs and ripostes, plus opening doors and operating devices. Doom 2016 and Eternal use paired animations for glory kills and chainsawing. Every fighting game in existence uses paired animations for throws/grabs.
So what’s bad about paired animations? Many reasons were listed in the Asscreed developer’s quote. Paired animations don’t let you hit multiple enemies with large weapon, because they’d need to make a specific animation that hit multiple enemies and a coded way to transition into it smoothly (Deus Ex HR did this, but it cut to black so it could set the enemies up, otherwise the transition would be jarring). They don’t factor in the length of your weapons, because most weapons have distance-closing animations, letting you snap to a target. And the speed of the move doesn’t matter, because you can’t be interrupted by the opponent you’re attacking, since they’re caught in the damage receiving animation from you.
The key problem with paired animations is the way that they snap onto enemies. Snapping in general can be problematic, because it erases specific circumstances, normalizing them into the same outcomes every time. Paired animations don’t care what was going on before they started, your spacing, velocity, movement, they take whatever happened and deliver a uniform result. Many paired animations are also invincible, because it’s difficult to resolve what would happen if they were interrupted. This leads to the awkward circumstance of trying to initiate a paired animation on purpose to go through other attacks, or coming out of a paired animation with an attack directly on top of you. One clever move for Nioh from Dark Souls, was removing backstabs and instead giving bonus damage to hits from behind. Parries in Nioh are still frequently paired animations.
When is okay to use paired animations? Paired animations are good for actions that specifically require them, obviously, such as grapples, and operating objects. In most fighting games, throws make both participants immune to damage. In the Smash Bros series, people can still be hit during a grab, but there is hyper armor applied during the throw part of the animation, to guarantee it finishes successfully. Paired animations are acceptable in circumstances where 1 tap will eliminate an enemy completely, since it would already always have the same result, and already doesn’t depend on circumstance. Examples would be takedowns in Deus Ex Human Revolution, or Glory Kills in Doom 2016.