The game plays a drumroll sound effect when you're guessing a word and have just one letter left to guess. If there are still two or more spaces left when you hear this sound, you know that those two or more spaces are filled with the same letter. So if you see "L A - - E R S" and hear the drumroll you can guess the word is "L A D D E R S" and not "L A Y Z E R S."
