Despite the custom deck Ratuki has a simple concept as its heart. Build piles of cards from 1-5 and the first player to lay down a five gets all the cards. To make this challenging there is a separate pile in play for each player, and when going from 1-5 you can play a card above or below the current card. This means piles can go up or down in number and players have to watch all the piles to wait for the opportune moment to play a five.
Not that you'll always have a five in your possession. You can only have three cards in your hand at one time, so you'll need to either play or discard a card to get another one. Interestingly despite only having five possible numbers there are multiple version of each number. So one player may have a four on the side of a dice, the other may have a hand holding up four fingers. There's not many varieties, but there are just enough to mean you have to think a little about what cards you have.
It's tricks like that which attempt to make the game frantic by confusing you. Ratuki plays out like a game of snap only without any set turns. Players can choose to wait it out with a five for the opportune moment to strike, or can lay down card after card. With two players this is quite strategic, and it's good fun to play a five just as another player is about to put down their own. If you time it right you can interrupt a players run by laying your card just as they go to lay theirs. Players have to pull their hand back between laying cards, which opens the door for interception.
To stop players waiting around for the right moment, a deck can only start on the number one, so eventually a player has to go. There's the Ratuki card to consider which can be any number you want. Other than that it's a streamlined card game, that does away with complicated rules in order to make the play as fast as possible. Players accumulate cards until they get to 100, and then it's back for another go.
With only three cards in your hand, and so few possible variants I still prefer Uno for a game with more depth, but Ratuki is certainly a fun alternate if you just want a quickfire card game and have had enough of Snap.