I have decapped a lot of people, I suck at explaining things, but I'll try my best.
So there are a few ways you can decap your opponent, one of which is getting your hand behind the neck of your opponent, and pretty much strangling their head off. Decaps involve a lot of wrist movement usually, the most effective way in my eyes is getting your hand behind your opponents neck joint and either contracting your wrist or extending your wrist, along with contracting your elbow and sometimes lowering your shoulder. I'll include a replay below (
TA Decap 1) where this is used. (I made the replay in single player as an example, I went by Aikido turnframes though, so you will be able to use that opener and follow up if you'd like)
Second way, hard impacts. If you are in a situation where you are rotating your body and your arm seems like it's swinging faster than a fucking Lamborghini, try to direct the impact point (Hand, wrist, forearm) towards your opponents neck joint. This is really effective in getting decaps, because it's so much blunt force hitting the neck joint. I have an example of this where I straight punch Uke's neck and decap him (
TA Decap 2) (I made the replay in single player as an example, I went by Aikido turnframes though, so you will be able to use that opener and follow up if you'd like)
A 3rd way of decapping your opponent, ghosting your hand under their neck joint. So if you're trying to decap your opponent and you find yourself in a position where your hand is ghosted under their neck joint, do the complete opposite of what your arm is at that moment. If your wrist is contracted, extend it, if your elbow is extended, contract it, If your hand is grabbed, ungrab it. This will cause your arm to pretty much pry the opponent's head off and I'd say it works 90% of the time if you do it right. I'll post an example of this below, (
koz vs remzombie 111) I got a really nice save by doing this.
Also I will post a replay where I get a knee decap, you can pretty much decap your opponent with any limb as long as it's moving fast enough.
There are many other ways of decapping your opponent, I'm sure others will post some other ways
Now, not a ton of players know this, or at least not a lot of players utilize it. If you contract your neck joint, it makes it probably 70% harder to get decapped. I don't know why it does, but it does. I always keep my neck contracted, and players that frequently decap people (Sparky, Fire, Nulu) can't decap me (unless they get lucky) So I recommend always keeping your neck joint contracted, it helps a lot.
Here are the replays