Humanoid robots aren't cheap. The lowest price kit runs at least $700, and builders typically pay from $1,200 just to get started in the biped robot game. So, when they compete at an event like RoboGames, they tend to be a little cautious about breaking servos or brackets, at least until the official part of the competition is over.
But, once they can relax and not have to worry about missing their turn in the ring due to a broken part, they throw caution to the wind. All the robots, both lightweight and middleweight, get into the ring together. When they referee shouts "Start!", the robot rumble is on.
The game is extremely simple. Knock your opponents out of the ring and avoid the same thing happening to yourself. The Last Robot Standing wins.