The game operates in turns, turn length in frames and planning time is different for each mod, and length of frames per turn can also change for each individual turn. For each turn you choose which joints to contract, like you could raise your shoulder, contract your pec, and contract your elbow, then about 30 frames later, you could lower your shoulder and extend your elbow to try to hit your opponent from above. This is a drastic oversimplification of what you'd actually want to do as far as hitting your opponents, but basically if you wanted to, you could do that.
With the mod Aikido (I think), the objective of the game is basically wrestling. You try to get your opponent to touch the ground inside the boundaries with something other than hands or feet, or get your opponent to touch the ground outside the boundaries with anything, all the while trying to make sure you maintain good leverage and a good position while not touching the ground with disqualifying members. There are mods with weapons and lower dismemberment thresholds than aikido, in which things become more either more complicated or more simple, depending on the other rules of the mod.
For instance with a sword mod in one game, with a single slash I utterly annihilated Sinister's left arm, causing 336000 points of damage, it was split into 2 pieces and detached from his body. In most games you do about 50000-150000 points of damage, depending on how active your striking was, which gives you a small picture of my amazement at my single sweeping cut.
Apparently there's even a street racing mod, where instead of bipedal humanoids, you're a car. I haven't tried that one personally.