Hinging occurs where the join between the fly line and the leader is inefficient. As Mark says, that may be as a result of a 'broken' joint, as in his example of the line covering cracking above the loop but more commonly the effect is caused by the difference in diameter and rigidity of the change from the fly line to the leader. Now, if you fish mainly with the wind behind you, you roll cast a team of flies over the front of a drifting boat for example, you'd have very little difficulty fishing with a level leader of say 4X, around 6lb breaking strain. However, if you were attempting to cast a large Mayfly upstream, facing a downstream breeze you'd soon discover that the leader would not extend and turn over as the energy transfer from the fly line to the 4X leader is insufficient - the cast collapses - the analogy would be along the lines of attempting to push a hair into play dough. Hence, the joint between fly line and leader needs to continue the taper of diameter/ rigidity/ density.
A belated welcome back mate. What I was hinting at above was that failures of turnover, actually caused by inefficient technique, are often ascribed to inefficient gear/tackle. I don't want to debate the science because imho in this context one aspect of casting mechanics leads to others and in the end complexity and confusion generate little practical light for the fly fisher.
Thing is with turnover, after loop formation there is both a push me (KE of the fly leg) and a pull me restraint from the rod leg and it's only when KE (or restraint) becomes critically low that the efficiency of its "transmission" via the loop becomes a biggie. To be clear the context I'm assuming is WFF or DT with a front taper followed by a leader, usually but not always also tapered.
Consider, a different but similar example, the problem of overhang in casting a shooting head. Too much running line outside the rod tip and it all goes brown. Is that an instance of (reverse) hingeing? Would increasing rigidity without changing linear density of the running line make a significant difference?
Let me suggest something else to ponder. Taking your example of an upstream cast into a downstream wind (assuming it's blowing steadily) would it make a difference if the 4X level leader was shortened, let's say progressively from 12' to 2'. If so is "hingeing" still the right way of describing the problem? Alternatively, if downstream is still south and upstream is still north and we gradually fan cast from south to west or east and on around to due north at what point would the hingeing trouble begin?
Like I said, good to have you back.
Oh and returning to the general question of connections loop to loop (and again avoiding a dissertation ) ime they don't come loose and kept small enough a braided loop doesn't cause a significant turnover problem, if any.