WARNING! Never do this, you will kill someone!
Push your hand in his face and when the attacker or opponent opens up the solar plexus, you would smash your heel palm (tip of ulna or Styloid Process) into the solar plexus. For the mma fighter you would punch through the solar plexus (affecting the liver/gall bladder/diaphragm) or you could even use a palm strike which I think is an under utilized tool in mma. Your Virtual MMA Coach and World Famous Self Defense Expert Coach David Alexander
Solar Plexus Strike by Megumi Fujii (see more here http://freemmaschool.wordpress.com/targets/)