This activity is meant to review songs with multiple verses.
The chorister calls out either a word or a theme which occurs in only one of the verses. As an example, if we were reviewing "I Am a Child of God," I might call out the word "parents," which appears only in verse one. Then all the children hit their fists on their open hands three times and hold up the number of the verse they think you are referencing. Everyone sings the verse.
As an option, I somtimes make word strips and have a child who is singing well choose the next word. Then I can play the game with them. My kids love when they get the answer right and I don't :).