What's the curve on the back of the chopper for?

Analyzing this cutting action, assuming the food is no longer being cut (rigid body) and there is infinite friction between it and the blade, the blade will be doing a pure roll on the cut marks before the food. A flat knife blade curve is something like an involute, a curved knife blade curve is something like a circular arc. That must be a shorter circular arc, with a shorter distance of wrist movement. And the small rate of change of the angle of the tangent of the arc of a circle means that the wrist moves up and down at a small angle, which makes it more comfortable and easier to deliver power. In this case, a curved blade works well.

The thin blade of a kitchen knife is sharp because it reduces the area of force when the same pressure is applied when cutting vegetables, thus increasing the pressure. To sharpen a kitchen knife, soak it in salt water for 20 minutes, then sharpen it, pouring salt water on it as you do so.