Pressing it when going less than 5 mph will cause a "parking-brake" action.
Pressing it when going over 5 mph will cause the engine to shift into neutral - then the driver can simply apply brakes and stop in a safe location.
Failing that, holding down the power switch for three seconds will actually shut the engine down completely.
This guy was a complete hoax.
What got me was the comment that the car eventually stopped when stepping on the brakes and applying the emergency brake at the same time.
Pressing the brake pedal actuates all 4 wheels. The emergency brake uses the same brake components only without use of the pedal. Applying both gives NO extra braking power over just the brake pedal.