I couldn't find any mention of this in the manual anywhere. Today, while I was driving at highway speeds and occasionally pressing the left stalk button in for a single wipe (because the auto wipers get totally confused in light snow), I accidentally pressed the right stalk button instead. There was a chirp of an alarm and, thankfully, nothing else. The car didn't try to put itself in park or even slow down in any noticeable way. Later, when I could safely try it without anyone behind me, I tried it again and saw that the screen was saying "Emergency brake activated" (or something to that effect). I found that a quick press of the button enables the emergency brake, but thankfully only for a split second. When you hold the button in, the e-brake will apply lightly at first, and then rapidly build to a powerful deceleration.
I just thought I would share this, as it could be handy to know in an emergency if the main brake system fails. Per the manual, "the parking brake operates on the rear wheels only, and is independent of the pedal-operated brake system." It's obvious from using the car and reading the manual that this button applies the brake to park, but it was not obvious at all (to me at least) that the button could be used in this manner while driving.