One of the 180 rules is a trajectory rule where the backcast target must align the forward target. The closer the front target the higher must be the rear target. I teach to “ring a bell” on the backcast and for the caster to imagine in his mind this bell behind him. In fact this is the very first thing that must go through the anglers mind when making a Snakehead shot (or in fact any cast really. It applies to taking shots, casting into the wind, accuracy targets). Very few casters seem to be able to raise the trajectory of the backcast so the the loop straightens above the horizontal, although they seem to have no problems doing this on the forward castSo it would be an inertial effect? Why not, but there is the rod leg to lift somehow.
With short lengths of line it is possible to break this 180 rule. I believe this Is because the tip path is the controlling factor in these conditions. With longer casts the angle of the backcast becomes critical. If the backcast is lower than the forward trajectory this can open the loop. I think there are times when instead of opening the loop it lifts the loop, this is a less extreme example. In certain circumstances it could even be beneficial but mostly I think you’re far better off to stick the backcast aligned to the front target. (I really want to make some videos of these aligned loops and will do so when Graeme is here).
In the case of a Roll Cast it’s physically impossible to align the backcast angle to the forward trajectory unless you are fishing for bats. This is one reason why for very short roll casts I would recommend tilting the rod plane close to the horizontal (the other reason being of course that it’s stealthier and helps keeps the rod out of the fish’s window of vision).
With regards the lifting or straightening of the rod leg I’ve always regarded this as a result of tension from the loop nose to the rod tip. There are also a few bumps/waves running along the rod leg - less with an HT6 of course and a shitload more with a TFO.
One experiment I would like to do is to buy, or preferably borrow, some roof guttering and lay the line inside this. It would be possible to set this up at a horizontal angle, level with the rod tip, and make forward casts starting with the line in the guttering. You could make slow and fast casts from an identical backcast position. Unfortunately I don’t have a hardware shop of my own but I know a guy who owns a very nice Chinese restaurant, who’s father owns a hardware shop! It just might be possible and I’ll ask the next time I order sizzling tofu.