From The Apple Wiki

Motion coprocessor marketed by Apple as M8 and codenamed "Phosphorus" which is built into the iPhone 6, iPhone 6 Plus and iPod touch (6th generation). It is used by apps that access motion data while the device is sleeping (such as Nike+ Move [Archived 2013-11-06 at the Wayback Machine]), allowing the main CPU to go to sleep and therefore conserve battery life.