There will be 4 letters flying across the screen. 3 are white; 1 is black. When any of the 3 WHITE letters is the letter A, press SPACE. Ignore the BLACK letter. If you get it wrong, the heart in the middle of the screen will get smaller and will become more transparent. Don't worry. Even if you haven't taken any damage you'll still be able to see through it. There's a time limit too. If it runs out, you'll lose, regardless of HP.
It's sort based off of Danganronpa's Hangman's Gambit.