As you may recall from our review of HTC's latest device, this mode will be automatically activated once your battery reaches a predefined level. When in the Extreme power saving mode, the HTC One (M8) is stripped down from a plethora of its battery-eating features and only allows users to access the most basic ones, such as call making, texting, and manually receiving emails. While under this mode, the smartphone will use roughly 1% of you battery every 3 hours, which is quite an impressive result.
Be advised that the update has started rolling out just today, so it may take a while before it makes its way to your new HTC One (M8).
source: Sprint via Android Central