I routinely practice the first go-round of anything without the left thumb at all, and put it back only after I've felt what it feels like to have core engagement while playing the passage. The thumb, at that point, seems more like a fine-tuning mechanism and passive lever than something that is used to squeeze with. But i have never found a way to do this while standing, so I sit. I think @Tom Lane 's explanation of John Goldsby'splaying position is like is good. I've watched John play for years, and it makes sense to me. I also notice that Rufus Reid and Lynn Seaton use their angled pins to achieve a seated-type posture while standing, which also resonates. We all prefer the posture that we prefer, but I think however you fall on that spectrum, minimizing left thumb squeeze is a good thing. As always, EEMWDSOCB.