While most power napping apps wakes you up after a predefined time, TouchNap uses you as input to know exactly when to wake you up. By using simple techniques, it will simply wake you up right before you are about to enter deep sleep. This is achieved by the knowledge that muscles relax more and more the deeper you sleep.
You hold the phone in one hand and touch the screen with two fingers, kind of like playing a guitar. If held correctly, when you go deeper and deeper into sleep, the muscles in your fingers will relax to a point where they let go of the screen and the alarm goes off. This is the preferred time to wake up from a power nap.