If my protag managed to get the person they’re fighting to kind of double over (probably by punching them in the gut) would it be at all realistic for them to break the other guy’s nose by slamming his head down while also bring long their knee up?
Yes, that’s an extremely common and effective strike combination. Well, the sucker punch, less so, but running a knee into their groin or stomach, then following with a quick knee strike to the face is. Face strikes are an excellent stun, and it’s more effective than relying on a knee to the groin incapacitating your opponent. There’s the added bonus of a lot of blood from the nose, but the real kicker for the knee strike to the face is the force generated by slamming their head down into the rising knee. The force comes from two directions, not just one. This is what makes the knee to the face more effective than the knee to other parts of the lower body.
You’re going to do more to them than break their nose. They’ll be dazed, disoriented, and bleeding profusely from their nose. They’ll also be crying, not from the pain but because a broken nose will do that to you. Depending on your character’s combat tactics and amount of time they have available, they’re free to bring the head up (because they’ve still got hold of it) and repeat steps two and three. Or, they may move on to working the body over because they’ve got the other character in a state where they’ll have difficulty fighting back.
A lot of combat combat works like this. Create an opening, and immediately exploit it before your opponent can react. This is how real combat works, but you don’t often see it from writers without a background in violence. They’re working off the attack, defend, attack rules turn based combat from RPGs, or the queing system from television. Real violence is a function of taking the initiative from your opponent and pound on them until they can no longer retaliate. Combination strikes, the process of stringing multiple attacks together into a cohesive combat strategy, are often difficult for the unitiated to wrap their heads around. For a lot of readers, thinking ahead in combat like this will impress them.
A lot of basic attacks utilize a simple one-two setup-exploit structure like the sucker punch with the follow up knee to the face. Getting your hands on your opponent’s head and bringing it into a solid knee strike is an effective tactic utilized by many different martial arts from around the world. The technique is also an easy one two the slightly better than average schoolyard bully can master.
The big thing with head strikes is making sure the bones you’re connecting with are more solid than the bones you’re striking. For example, you do not want to strike the forehead. It’s basically just a large, heavy, bone plate. Either side of the forehead, where the plates meet, is an excellent target. You’re aiming for a structural weakness. The face is made up of a lot of relatively fragile structures, with a chunk of soft cartilage for good measure. Your thigh, in contrast, is a massive, solid, load bearing, bone, reinforced with heavy musculature. Yeah, hit them with the end of that. Their face is way more fragile. (Your kneecap should be pretty securely locked in place when you connect, unless you’ve got some serious medical condition.)
This is the same danger with punching, and other hand strikes. Your hand has twenty-seven delicate bones, they allow for fantastic flexibility, and utility, but your opponent’s skull has eight fused plates, and fourteen in the face. Still want to punch them there? Personal advice, aim for someplace softer, or use something better suited to abuse than your hand. (Your knee is much better suited for that.)
So, let’s talk about combat inertia for a second. This is an abstract concept, but it’s a way to explain why combos like this work. It’s easy to get into a “turn-based” view combat. Your character takes an action, their opponent acts, your character acts again. In the real world, combat rarely works like this unless both fighters are completely exhausted.
Combatants are often looking for ways to exploit their opponent’s defenses, find/create an opening, and use this lead in as a launchpad for their real attacks. Tagging someone in the groin, or in the gut, won’t incapacitate your opponent, but it can buy you a little inertia to follow up. Usually, a knee to the face won’t put your opponent down. But, use the first to transition into the second and you’ve bought some time. No turn for them right now.
Here’s a fun thing about this specific combo. You can do it from nearly anywhere. If you’re on either side of your opponent, or facing them, you can deliver a quick strike to their gut, and follow it with knee strike. Usually from one knee strike to another, though punches and elbow strikes can certainly get you started depending on the exact positioning. Anything that will make your opponent double over. Depending on placement, this can smoothly transition into a number of chokeholds. This is a tool, and it can be mixed with fluidly mix with other options. The sucker punch to the stomach, into a knee strike to the face, can easily transition into a guillotine choke when forward facing or a standard triangle choke from the side. The strike can transition into a push kick, such as sucker punch, knee strike, push kick to the stomach where you crank the knee to your chest and shoot your foot outward. Or, simply rinse, lather, repeat with the knee to the face until they can’t stand up anymore.
Also, inertia doesn’t mean you have to stop at two strikes. You can keep going until you wear yourself out. The important thing is being efficient, and not getting into a situation where you’ve exhausted yourself against a fresh foe.
Combining, or chaining, moves like this is an important part of hand-to-hand combat. Surviving a fight means keeping control of a situation, and refusing to let your opponent(s) do the same. Preventing them from ever attacking, because they’re too disoriented by your attacks is a good way to do exactly that.