A 5 Watt charge can be generated with a very easy pull of the string. It doesn't matter how fast or slow you pull, the same charge is generated. And I'm told it gives the same charge as if you were plugging into a wall, so 1 minute of pulling is equal to the same amount of talk time you'd get after 1 minute of wall charging, or about 5 minutes of talk time. It's intended as a solution for quick emergency charging.
YoGen also demonstrated a foot-powered charger for laptops. Link -via Digg