The app actually monitors the battery temperature (exposed on all Android phones) and throttles down when it exceeds a certain threshold. The charging circuitr

The app actually monitors the battery temperature (exposed on all Android phones) and throttles down when it exceeds a certain threshold. The charging circuitry on some phones (not all) actually disengages the battery from the charging circuit path once 100% AC power is reached. In other words your phone's CPU runs off of AC power (and not via the battery) once 100% charge is reached. Why you would sign up is where it gets interesting - the app works via a similar incentivization scheme as how bitcoin miners work - the more you run the app (mine) the likelier you stumble upon a valid proof of work, and you get rewarded by virtual currency.