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:
(Image taken from this post and similar can be seen here and here and here.)
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?