Depends on the situation, really. If I'm on the defense, I keep my eyes on the opponent. If I'm on the offense, my eyes goes back and forth between my opponent and my own character, in order to see if my input was correctly recognized by the game, at the same time as I keep an eye out for holds from my opponent. I also watch out for walls and other danger zones during my offense in order to know what to follow up with. During spacing, I keep my eyes going from my character, to my opponent, then to the space between us, and repeat until I have to get back on either the offense or defense.