What actually is this feature & how does it work?

Its the 1st time im seeing such things & am very intrigued by it. Was in my boss's BMW & everytime the car comes to a stop the engine would die off, but when its time to move off, it would automatically start up again. I dont know what she (boss) does....maybe release the brake pedal or something &... show more Its the 1st time im seeing such things & am very intrigued by it.

Was in my boss's BMW & everytime the car comes to a stop the engine would die off, but when its time to move off, it would automatically start up again. I dont know what she (boss) does....maybe release the brake pedal or something & it wld just fire up on its own. But apparently you can disable this function by pressing a button near the ignition.

Is it some fuel saver or emissions thing or a euro 4/5 requirement??
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