That isn't about pressure. The screen does not directly detect how hard you press your finger. Instead, as you tap the drums (or whatever), the iPad calculates how much the accelerometer moved and judges how hard you must have hit the screen. With an actual pressure-sensitive screen, it would be far more accurate but also about more just than taps and hitting the iPad screen. For example, an artist "painting" a picture could shade lightly then press harder for darker areas or outlines. It would also be useful in gaming, where most modern consoles feature pressure-sensitive buttons on controllers now.
