Chi does exist. It basically means life force energy (as far as I'm aware). Whether it can be used in combat, I'm not sure but it wouldn't surprise me. Many cultures have believed for thousands of years that there is an energy system in & around the body. There are 7 main points running from the groin to the crown of your head (google 'chakra' system). Each are interlinked and are developed at different stages of your life. This energy is believed to extend outwards in magnetic sort of loops, that can be focused on specific things, given the correct training. It's also believed work on this system & each chakra point can effect your well being & character. With this in mind, if it can be used to heal, then I'd imagine it could be used to damage. From what I've heard though, to use it against someone in a negative fashion also serves to harm the self.
I await all the neg reps lol.