How to turn on "Hey Siri" when plugged in, but not always on
"Hey Siri" is now always on, if on, because of the new M9 co-processor; the same one that records your steps. Apparently it can do this without unnecessarily flattening the battery.

See the report from Apple Insider - Apple's efficient M9 coprocessor will let your iPhone 6s track pace, make 'Hey Siri' always on


  In a first for the iPhone series, the iPhone 6s will measure a user's walking or running pace when they exercise, without the need for battery draining GPS. Previously, this functionality was limited to the S1 chip in the Apple Watch.
  
  The M9 also allows for Siri to be always on. That means users will be able to use the device's "Hey Siri" voice command at all times.