This knockdown manoeuvre mechanic is an actual house rule mechanic that we use in our own Dungeons & Dragons games. Specifically:

Combat manoeuvres can be cool and interesting, and add tactical choices and challenges to a fight. But the game mechanics of combat manoeuvres can be clunky and complex. They can suffer from rules overload, in that some systems try to define a specific manoeuvre for everything. There might be a rule for disarming an opponent, another rule for knocking someone off a horse, another rule for pushing them backwards (into a pit or a fire, for example), another rule for knocking someone down, another rule for shooting an arrow in the eye of a basilisk to blind it, another rule for trying to cut off someone's hand, or a monster's tentacle... you get the idea. There might be a rule for 20 or 30 different things you can do in combat - but at the same time, if you want to do something and there's no rule for it, you're stuck. The game doesn't support it, so either you can't attempt it, or the GM has to make something up on the spot.

The solution to this? Combat Manoeuvres the Easy Way!

Here's how it works:

1. Player decides they want to try some interesting combat manoeuvre. It could be any of the things listed above, or something else. The only limit is the player's creativity.
2. Player makes a hit roll as usual.
3. If the hit is successful, the person controlling the defending combatant gets to choose to take either the effect of the manoeuvre or regular damage. (For monsters or NPCs, this will be the GM.) If you use critical hits (we don't), the defender gets to choose to take either the critical hit damage, or the manoeuvre.

That's it! It also works for NPCs or monsters attacking the PCs. The GM decides the evil priest wants to bash the wizard in the arm specifically to get him to drop that pesky magic wand. GM rolls a hit, and the wizard player decides whether to take mace damage or drop the wand. In practice, the GM and players should make the decision based on the motivations of characters/monsters in the fiction. For example: You swing a sword to disarm someone to try to de-escalate a bar room brawl. The defender may prefer to (a) stay armed because they're raging mad, or (b) avoid damage because they realise you're serious and they're in over their head.

The system is:

• Simple. It only adds one tiny rule.
• Flexible. That one rule covers any possible manoeuvre you can think of.
• Unabusable. If a player tries an overpowered manoeuvre (e.g. instant decapitation), the defender just takes regular damage.

This rule encourages cool, creative combats that aren't just slugfests. Instead of just rolling 50 times until the dragon is whittled down to zero hit points, attackers might try to blind it, or stuff a cloak in its mouth so it can't breathe fire, or chop off a claw so it can't attack, or force it back to a place where they've set up a net to drop on it, or throw a chain around its neck to pull it into a cage, or goad it into chasing the rogue so everyone else can fill sacks with treasure before it returns.

Credit where credit is due - we didn't come up with this rule ourselves. I first saw it on the Odd Skull gaming blog, and Odd Skull credits a couple of earlier versions of the same idea, on 4d Caltrops (unfortunately I can't find the exact post) and Tales of the Rambling Bumblers. It's worth reading Odd Skull's and Bumblers' explanations of the system too, for more context and ideas.

Transcript

Luke: Your plan is too risky. You need more training first. See you in the morning.
Rey: Wait! Stop!
Luke: I head back to my hut.
Rey: So his back’s turned? I surprise attack from behind, attempting to knock him down with my staff!
GM: Uh, really? PvP? Okay... attack roll.
Rey: 16!
GM: Corey, choose damage or being knocked prone.
[SFX]: Whack!
Luke: I’ll take the knockdown.
Rey: You’re coming to see Kylo Ren as my prisoner, whether you want to or not.
[SFX]: ke-raack! {lightning in the stormy clouds above}
Luke: Stop! This is horrible!
Rey: Realising you’re old and no match for my sick moves?
Luke: No, you’re railroading me without even being the GM.