Here's the (silent) promo video - you'll get the idea. Fuel cell in one side, some water(!) in the other and some clever 'proton exchange' technology (see quote below) in between and you've got charge being generated:
How does it work?
"To operate, hydrogen must be supplied to the fuel cell, and the fuel cell must be exposed to the open air. The fuel cell inside POWERTREKK™ Charger silently converts hydrogen into electricity via the Proton Exchange Membrane in the Fuel Cell."
The reliance on buying and storing the fuel cells does make the concept a bit odd, mind you, since you could just as well buy some spare batteries for your phone and take them along. Or buy a load of AA cells wherever you are and use an accessory to convert these for USB use. Or take a standard portable charger and top it up from mains or solar power when available. With the Powertrekk, when the fuel cells run out in the middle of nowhere, you're basically stuck, since you can't exactly buy them at the local rural store....
The charger does at least have a backup store of 1500mAh for times when the fuel's run out, but note that this isn't a lot, not enough to charge most smartphones once.
More at the Powertrekk web site.