Unfortunately not much information was given out by the NTT reps, but at the moment the battery (which has been dubbed the “Ultra Rapid Rechargeable Battery”) looks like a lithium-ion battery sleeve that seems to be available only for the NEC Medias Android smartphone, which is currently limited to just the Japanese market. This quick charge appears to be achieved by pumping more power through the battery than normal, thus achieving a faster charge speed. The capacity of the battery is low too and does not look able to charge your handset back up to full (which charges at regular speeds).
However we’re thinking the idea here is that because the sleeve itself charges so fast, you will be able to continuously keep it topped up (assuming you have access to a power source) and make sure that even though your handset’s battery isn’t on full, it isn’t dead either. No word on when NTT plans to release this battery pack or if they have plans to make it compatible with other handsets.