He is using that position as a convenience because of the shape of the rotor If you lay the rotor so the flatter sides are top and bottom, the keyway will be at either 11 o clock or one o clock. The correct position is at one o clock. Because the engine fires every revolution, you turn the crank so that keyway is also at one o clock and install the rotor. You could use 5 and 7 o clock if you wanted to, but then it gets confusing. If you turn the crank and rotor so everything is at one o clock, and the rotor is positioned as illustrated, the indexing will come out every time. Hmmm. Now I'm confused.
