Having started my fly fishing journey with spey casting, I have imagined this technique of casting to be very similar to the moves in a (straight-line sweep) spey cast: sweep, drift to key position, fwd cast. Perhaps that is why I find this style more effortless than vertical overhead, especially due to the ease of incorporating smooth power from elbow movement on the backcast. And perhaps, just perhaps, because of this relationship I have it approximately correct. But then there is the issue of tracking "well enough" ... for certain testing situations.
Some instructors teach spey casting by starting with an elliptical cast and then letting the anchor land.
This is a can of worms. Instead of getting my hands all dirty, I can just lift the lid. Here is the FFI MCI on the water -tasks video section on D- vs V-loop:
And here is a much longer video of two ways to cast a skagit line:
I have played with these "constant tension" casts. I certainly can not do them for all cast/line length combos. For example, with an SH rod, and any longer line, I can not do a clean D-loop snap-t. I admit that I have only tried with incline sweep. I would find it weird to dip during the sweep, but perhaps that is what it takes. Still, even in this case, I at least imagine that striving towards a D yields better lifting power (for heavy flies / sink tips). Even if the result is not a perfect, clean D.
For some other casts, now that I have decent video equipment, the jury will soon give a verdict.