It's not the stomach, itself, but the solar plexus. It's frequently taught in Karate (that's where they aim at when they do a simple straight punch), because when you get hit in the solar plexus your diaphragm is unable to move for a moment, which means so does your lungs. You're supposed to lose air and lose consciousness for some time if done right.
But yeah, that's what it is. A single punch to the solar plexus actually WILL knock you out in 1 go if you hit the right spot (and with enough force). It's amusing how media always portrays strong dudes as taking a hundred hits before going down...but the human body has weak points. If they get hit...you go down. Unless you've got a suit of some sort of super-material that absorbs force and strengthens bones and whatnot.
The cyborg riders were more plausible than some of the Heisei riders in this regard.
It's not the stomach, itself, but the solar plexus. It's frequently taught in Karate (that's where they aim at when they do a simple straight punch), because when you get hit in the solar plexus your diaphragm is unable to move for a moment, which means so does your lungs. You're supposed to lose air and lose consciousness for some time if done right.