Hey all, played D&D a long time, but not for 20 years now.  Recently started watching some youtube stuff and wanted to share some old homebrew rules we had.

Carry Through Damage -- when a truly great blow hit a less than great target, it splatters the victim allowing for excess damage to carry through to the next target.

Damage in excess of 2x the remaining hit points of the creature being struck is carried through to the next creature assuming it is within range.

Example:  A fighter scores a natural 20 + strength bonuses etc on an orc that only has 4hp left.  His blow does 18 damage.  4 hp goes into the orc. 4 hp gets spent carrying through the orc, the remaining 10 points of damage hits the next orc.

We would use this in a scenario in which a mid to high level party is taking on a large number of lesser monsters, where pretty much any attack is going to be a kill. It allows the melee classes to feel like they are doing more than just one hitting everything.  Also helps mitigate the "You just did 20 damage to a goblin with 2hp left" situation, so it doesn't feel like an epic hit is wasted.

The 5e DMG actually has a different optional rule for it:

Cleaving through Creatures

If your player characters regularly fight hordes of lower-level monsters, consider using this optional rule to help speed up such fights.

When a melee attack reduces an undamaged creature to 0 hit points, any excess damage from that attack might carry over to another creature nearby. The attacker targets another creature within reach and, if the original attack roll can hit it, applies any remaining damage to it. If that creature was undamaged and is likewise reduced to 0 hit points, repeat this process, carrying over the remaining damage until there are no valid targets, or until the damage carried over fails to reduce an undamaged creature to 0 hit points.