For example;
In the classic "Key Word Game" the students would choose a word from the topic.
For example: The topic is "colors" and the key word is "blue".
Then the game would go something like this.
ALT: Red, clap, clap.
Students: Red, clap, clap.
ALT: Green, clap, clap.
Students: Green, clap, clap.
ALT: Blue (students try to catch the eraser). The student who catches the eraser is the winner.
In this variation it's a little simpler. You would only change the topic.
Let's say that the topic is colors.
ALT: Red, clap, clap.
Students: Red, clap, clap.
ALT: Green, clap, clap.
Students: Green, clap, clap.
ALT: Soccer (students try to catch the eraser). The student who catches the eraser is the winner.
Because soccer is not a color that would be the key word that shows it's out of topic and in this case they would try to catch the eraser.
That's it. Pretty simple but a little interesting I suppose.