I’m not sure if this is what the OP was referring to but it fits;
This is what I was taught. We used to do this all of the time. These days you are lucky to find anyone that remembers… Well I digress!
I believe your instructor was referring to and Open Wye Open Delta; The lighting pot (or larger pot) should be on the leading phase. It has to do with imbalance and inefficiencies in Open Wye-Delta banks. Think about it a bit and it will come to you!
To determine if you built the bank correctly, maps are correct, blah blah blah!
Mark the high leg blue for “C” phase. You might throw some orange tape on it as well. This marking will not change and was once industry standard…
Then;
Put the “C” phase lead from your rotation meter on the high leg.
Put the “B” phase lead from your rotation meter on the leg tying the x1 and x3 of the two pots.
Put the “A” phase lead from your rotation meter on the last leg (single on the lighting pot).
If you get clockwise rotation, the lighting pot is on the leading phase. If not, you can rearrange the secondaries, swap primaries... Well, I won't tell you how to do your work, just how to do it right!