AnandTech | The HTC One (M8) Review - http://www.anandtech.com/show/7893/the-htc-one-m8-review
Extreme Power Saving Mode
In the Extreme Power Saving mode you’re locked out from all but five apps (phone, messages, HTC Mail, HTC Calendar and Calculator ).
The display brightness is clamped to 170 nits and max CPU/GPU frequency is limited to 1.2GHz and 320MHz, respectively. All four cores remain capable of being plugged in.
Background data is also restricted - only SMS/MMS and phone calls will come in automatically. Emails need to be manually refreshed, and all other apps are quit upon entering the mode. The sensor hub is powered down, which disables features like the Motion Gestures and Pedometer.