There are a number of classic balance toys that illustrate how this stuff works.
You can even have your center of gravity below the point of the support (and a very small point of support at that), and still have a very stable point of equilibrium, even if it looks quite hair raising & impossible.
You can balance a wood toy with a floppy belt jammed into it on the very tip of your finger.
This is a scout craft called a Sky Hook.
If you are interested in this stuff, the place to look is Classical Mechanics textbooks. Feynmann's is particularly charming.
Landau & Lifschitz is still excellent.
Tessa has great core strength. She just doesn't need to use it when doing a transition through a sitting position on Scott's shoulders.
But that's not the point.
The point is, that the rules on lifts have been applied in some very inconsistent ways, including to Tessa & Scott this season, but also to less renowned teams, like Guignard & Fabbri.
I'd like to see the rules made more clear. I don't see who it hurts to define "sitting" and "shoulders" a little more clearly. It certainly wouldn't hurt Tessa & Scott, because they are not using any lifts with anything like sitting on shoulders this season.
But it might help Elena & Nikita, and for that matter Charlene Guignard & Marco Fabbri to have a second's "transition" allowed for sitting, just as is given for gyno lifts. I'd like their lifts to be explicitly legal. I'd like Tessa & Scott's Carmen lift to be explicitly legal. They are all great lifts, and I'd like to see them.
I don't see why anyone would disagree with that.