I am winding an EFHW matchbox and many of the designs I've found (including the diagram provided with the kit I bought) include a sort of "twist" when winding them:
My understanding is that the section where the two green turns go with the initial red turns is called a "bifilar" winding and some recommend twisting the two wires together for that part — its own topic that I can research a bit more on my own.
But what is the trick where the red winding crosses across the middle and then continues winding? Does it have a name? Does it count as a turn — or a "negative turn"? (In the diagram I have linked there are drawn 8+7 turns labeled as 2x7…but I suspect this is just a mistake.)
Why would the toroids be wound this way in two groups instead of just a simple total number of windings spread evenly around? When is this technique useful?