Some skills have a bonus effect labeled "QTE". When one of these skills is used, a sequence of arrows appears on the screen. Pressing the arrow keys (or the the equivalent WASD keys) in that sequence grants the bonus effect. Pressing the wrong key sends you back to the start of the sequence. The first QTE bonus you get (at the second level of your second skill) only requires you to press one arrow key; for those skills, mashing all of the arrow keys will suffice (this can be useful for the first hit of a battle, when the screen is still dark).