it's the shot that you don't see coming that k you tfo, or in general that your eyes did not pick up on the punch which leaves your brain unknowing and susceptable to shock when the blow does land.
don't get me wrong though, you can get ko'd even when you watch it coming in. From these types of shots you will most likely be able to atleast make an attempt to get up, your mind some how understands what is happening. But with the ones that you don't see they will simply leave you laying flat on your face with no chance of beating the count.
It's a defensive mechanism in your mind that when shocked or surprised from a blow will just shut your whole system down, kinda like hibernation mode filled with odd dreams and visions.......... Rockin'