Speed Zendo is a variant of Zendo. The setup is the same, except that players are not given answering stones. At any time, you may make a new koan, and the master will mark it. At any time, you may say "Guess!", and all action stops. You must quickly (within, say, 5-10 seconds, at the Master's discretion) state a guess. If it's correct, you win and the game is over. If it's incorrect, the Master builds a counter-example, and you may continue to take guesses as long as you can come up with them quickly. When you're done guessing, the Master gives you the Meditation Stone. You can't take any actions while you have the Meditation Stone, but the Master will take it away from you as soon as any other player builds a koan or takes a guess.
Although it's called "Speed Zendo" there is not actually a need to rush to create koans. A student can build no koans at all, see the pattern, take a guess, and win. Games do take less time because students who don't know what kind of koan to create don't need to, and students who have several interesting experiments can just run them. And if you have an idea, you don't need to wait for your turn to take a guess. Perhaps "Turnless Zendo" would be a better name, but that's not as sexy, is it?