The button must be pressed within two frames of pressing df, making this a difficult maneuver to do. You know you've done it right, because characters who can do it (Kazuya, Jin, Heihachi) will say something (Kaz, "Dorya!" Jin and Heihachi grunt). You will also know because there is more electricity flowing around your character than with a WGF (Wind God Fist).
Furthermore, you have the frame advantage even if your opponent blocks, since it pushes them away from you, and you recover while they are still in blocking animation.