The Extreme power saving mode arrives to the Sprint HTC One (M8) thanks to an update11
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