It always seemed to me mechanically inefficient. This was striklngly illustrated by an archaeological article on slinging and hillforts, where a series of drawings broke down the movement (the drawings showed a bearded man, presumably the author, in a sort of tunic). The end movement and release, with a sort of raised shoulder and hopeful upwards movement of the whole body on tiptoes, were familiar to me from experimentation, and particularly reminded me of how weak the form is at converting body movement into forward release and speed. I seem to remember the underarm form used by other specialists (Thom Richardson), with bad results. The idea that this would have been used historically struck me as unlikely. In contrast, helicopter, sidearm, and fig-8 all converge into effective release in the final stage. They are all attested historically and ethnographically. Is there any evidence at all for underarm being used in such contexts ?

I also find timing of release more problematic with underarm often resulting in ridiculous upwards shots, difficult to adjust. And I speak as someone whose shots aways drag left and high.

These two reasons (mechanical weakness; problematic release times) are why I don't sling underarm, preferring helo (before shoulder issues and in spite of slack issues), "Makron" (one rotation from body facing away), and now sidearm (vertical rotation(s) and side-release).