Sounds like a neat novelty cast but with all due respect I have a hard time envisioning a fly that flips upward at the end of turnover as a solution to overhanging obstacles.
Historically, the term "underhand" generally refers to two-handed casts with compact quick-loading lines and a distinct inward pump of the lower hand to snap a sharp cast out, this term most often coined by Henrik Mortensen.
On that note for obstacles behind, nothing beats a Spey cast which is essentially a roll cast with a change of direction and maximum efficiency combined with proper mechanics to reach full casting distance.
However I could see how an inverted loop could be also coined "underhand".
Knowing that any variance in the path of the rod tip acceleration affects the loop shape I think I can guess what you're doing. Dipping and widening causes tailing loops, collisions or big loops.
It sounds to me like you are dipping thus opening the loop (because it's inverted), then laying it as you would snap a towel at your buddy as kids (sans the whip-crack) nearly extended into position managing just the last forward reach of the line and leader into said tight spot.
It seems that a skip would "kill" the final extension into said tight spot so it does not make sense to do both at the same time unless one is very lucky.
If that final extension after the layout of the big loop is accurate, I could see it finding it's way into small spots.
Distance would be nearly impossible based on dissipation of loop energy. Collisions and tailing likely, and ground clearance (depth of wading etc) would be limiting factors.
I'm not convinced that a Lefty Kreh loop would be less accurate, or a Simon Gawesworth Spey cast less space-consuming but neat to think about just the same.
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