Reduce Motion will disable parallax effects, which do use additional resources to process the phone's motion, and these effects are turned off when low power mode is enabled. However, turning on Reduce Motion will do so system-wide and will also disable the new iMessage effects. You can leave Reduce Motion off but set your wallpapers with Still instead of Perspective to get a similar improvement to your battery life. Your iPhone may also be processing motion for other reasons, and if so Reduce Motion will not affect this and therefore have very little to no impact on battery life. The iPhone 6s is extremely efficient at processing motion, so this was more of a concern for older devices when the parallax effects were first introduced. Apple's tips on maximizing battery life here: http://www.apple.com/batteries/maximizing-performance/