To do that, you need to go to battery saver option in settings, now swipe to the usage tab, there tap on the option to show app that can run in the background. (phoneradar.com)
These include Google Now on Tap, Doze mode, improved app permissions and smarter battery saver option. (techone3.in)
Android's new battery saver option also isn't a part of the HTC update and we're not surprised: HTC already had a better system and it remains the same as it did before, where you're able to specify what elements battery saver will use to conserve power. (pocket-lint.com)