It doesn't matter. This will produce three flat surfaces. It looks like he's just using the word control to denote whichever plate is on the bottom. The top pla

It doesn't matter. This will produce three flat surfaces. It looks like he's just using the word control to denote whichever plate is on the bottom. The top plate will become concave and the bottom one convex due to the overhang of the top plate as it is being ground, but that is not relevant here.