The reason for this is because humans are actually capable of functioning for about three seconds after things as extreme as decapitation or disembowelment alone and those few seconds are just long enough for them to have the last laugh at your expense.
Skeletal muscle (as opposed to heart and smooth muscle) are completely dependent on signals from the central nervous system for every movement except basic reflexes. This means anything done with purpose requires real time updates and commands from your brain. As soon as the brain's signal is lost, the muscle relaxes. This occurs virtually instantly. Decapitation results in immediate, irreversible loss of controlling signals to all skeletal muscle. The body drops like a puppet whose strings were cut. Any further contractions of skeletal muscle are spasms. Given how quickly the limited loss of blood flow from a rear naked choke can cause a person to pass out, the loss of all blood flow would likely mean the brain in the severed head would lose consciousness very quickly. A beheaded individual will not take a single step; humans don't behave like beheaded chickens. Being disemboweled is survivable and as long as the deep abdominal aorta and renal arteries aren't destroyed, the blood loss may be much less than one would expect. Before surgery, the survivability would be zero, but the kill wouldn't be instant. With surgery, survival is possible but certainly not easy and likely to include severe residual problems. Continuing to fight while disemboweled would be extremely unlikely but perhaps possible. Claims of human beings taking even a step after being beheaded are pure Bullshido.
All that said, involuntary muscle spasms from a guy holding sickles or knives is still probably something you wanna avoid...
Moved thse justifying edits here