If you asked ten people what you call this game you'd probably get ten different answers, but the name I was taught for it was Issesse ( いっせっせ ). Maybe that's the Nagoya area's preferred name. It's basically a number guessing game, which gets harder the more the participants.
The break down is...
Number of Players: As many as you want. Our example will be with two players.
Objective: To correctly guess the number of thumbs which participants will raise.
Hand Position: Ball your fists with your thumb laid flat over the side of your index finger (as if you were preparing to give someone a thumbs up). Place your fists together, inner wrist to inner wrist.
Procedure: In your head, guess the number of thumbs that will be raised between yourself and your opponent. Unless there are some extenuating circumstances, you will have four thumbs between you. This means you can guess anywhere from 0 (thumbs raised) to 4 (thumbs raised).
When you have a number in mind, say carefully "いっせっせのX", where X is the number which you have chosen. At the same moment you say the number, both you and your opponent would have chosen to raise/not raise one/both/none of your thumbs. Count the number of thumbs raised between you.
Winning: You win if you guess the correct number of raised thumbs. You may drop one of your hands behind your back. When both of your hands drop out of the game (when you are correct for two turns), you are the champion.
Note: When someone successfully drops their hand from the game, re-evaluate the number of thumbs. If you have two people playing and one has correctly guessed once, then you only have three thumbs between you.
When playing with more than two people, there's not really a winner. It's more that whoever is left at the end of the game with a hand or two in is the ultimate loser.
You can play this even with people who don't speak Japanese. I taught my little brothers. We still say いっせっせ but instead of いち！etc we say the numbers in English.
Here is a random video I found showing a game in progress...