Cut out a circle 21" diameter
Place it on your grass, dead level
So the blade will cut at exactly the same height all the way round
Now hold it front down and push it slowly through the grass
That is where the blade will cut the grass, the first cut will be the last because next time the blade passes it will be passing over where the blade of grass was.
Now hold in back down and push it through the grass.
Note as it moves forward it will cut then recut then recut then recut remembering the blade is doing around 32,000 revolutions and there are 2 cutting edges so 64,000 times a minute a cutting edge passes over the circle you have in your hands.