You look in a bike mirror for one reason: to decide whether the car behind you is far away or close. The shape of the mirror decides how reliable that decision is.
What a convex mirror does
A convex mirror curves outwards. It fits a wider view into a small surface, which is why some small bike mirrors use it. The price is scale. Everything in a convex mirror looks smaller than it is, so a car seems further away than it really is.
What a flat mirror does
A flat mirror shows the world at its true size. A car behind you appears at the same scale as the cars you see in front of you. Your eye has no refocusing to do, and you can judge distance and closing speed the way you do ahead.
The usual objection: a flat mirror is narrow
That is true if the mirror is fixed on the handlebar. A fixed flat mirror shows one slice of the road. A mirror that moves with your head solves this. A small turn of the head sweeps the view across the lane behind you, and every position in that sweep is shown at true scale.
How the Head-Zup mirror applies this
The Head-Zup mirror is made of optically flat, distortion-free polycarbonate, and it sits on your glasses or helmet. Slight head movements let you scan a far wider field than a fixed handlebar mirror gives you, and what you see keeps its true scale. It weighs under 9 grammes and detaches at the slightest impact.