Baseball players talk about how they know they just hit a homerun at the point of contact. Basketball players can often feel whether or not a shot is going in at the point of release.
Does the same happen for a fighter as soon as they land a KO strike?
Ask Mark Hunt 
I know when I land the KO shot, I'm unzipped and ready before you hit the ground turdboy
Throwing a perfect punch can be like trying to hit a home run occasionally. . Freddie roach teaches his left hook on the speed bag, because if thrown correctly it'll jam to the board and won't bounce. Punches thrown on someone's jaw have a tendency to move the jaw more rather than the hand receiving the impact (since it's a stronger structure). That way I can tell if I have landed a bomb by the feedback I get from the punch (usually I barely feel anything).
I ask because I have never KO'd anyone.
Some call it a butter punch because you don't really feel the impact. 
GOATherder - Throwing a perfect punch can be like trying to hit a home run occasionally. . Freddie roach teaches his left hook on the speed bag, because if thrown correctly it'll jam to the board and won't bounce. Punches thrown on someone's jaw have a tendency to move the jaw more rather than the hand receiving the impact (since it's a stronger structure). That way I can tell if I have landed a bomb by the feedback I get from the punch (usually I barely feel anything).
I can attest to that in the home run comparison. When you barely feel the contact is when you know you got it perfectly. The energy is all put into whatever is being hit.
Nothing feels better than hitting a ball and knowing you launched it. You can feel and hear it when it's right on the sweet spot... There is no vibration in the bat, just a single crack... It's Perfect....I miss that feeling lol... I've dropped someone with a hook one time but honestly don't remember what it felt like, I just know after I felt like Chuck Liddell! Haha
I asked a couple of guys about this very thing for a story I did a while ago on the art of the KO (Link)
"It doesn't feel like anything." - Johny Hendricks.
"It's like nothing. It's surprising, because you throw and you feel a connection, but it's like a flash. Nothing. And then you see them crumble." - Michael McDonald
It feels like punching a shower curtain.....or a pillow......or lanolin
GOATherder - Throwing a perfect punch can be like trying to hit a home run occasionally. . Freddie roach teaches his left hook on the speed bag, because if thrown correctly it'll jam to the board and won't bounce. Punches thrown on someone's jaw have a tendency to move the jaw more rather than the hand receiving the impact (since it's a stronger structure). That way I can tell if I have landed a bomb by the feedback I get from the punch (usually I barely feel anything).This. It almost feels like hitting clean air.
feels like a bag of sand
At 3:07 i hit him and it felt like it went through his temple rather than a solid connection, i was surprised it koed him, my knuckle hurt a bit thats all.
Interesting... thanks guys.
What you want to do is turn the head faster than the brain inside it.
Many ways to make it happen and they can vary greatly from each other.
Last time I KO'd someone I didn't feel a thing.
I did need a new bumper though.
Only KO'd one person in my life. Was completely reactive to the situation. ..didn't feel the impact at all.
When I head kicked people, when I connected flush, it felt like a hot knife through butter. I still remember their knocked out bodies just falling to the canvas.
It feels just like any other strike thrown with intention, except that was the one that landed.
I was sparring last night and flashed my partner on accident and even though I hadn't had that happen before but even the second I connected and he hadn't gone down yet I knew that shot was gonna do it. It was a weird feeling, I hope to get it again in competition.