The semantic bases of such usage seems to the degree of bodily contact with the substratum, vigilance, engagement, etc. Flying/jumping means ceasing some activity of interest entirely and going off to do something else rather suddenly, standing implies readiness to do something else as needed but attending to the activity when one can. Sitting is regular involvement in the activity, though not to the exclusion of other things that need doing. Lying (not just on but also within) is deep involvement, almost to the exclusion of other activities (English 'immersed in', being 'wrapped up in', 'in a rut' 'be up to one's neck/ears in', 'in over one's head', 'buried in (as work)' etc. have similar import). Relative height is another way to look at it- we say we are 'over' with something to mean 'done with' (equivalent to the jump/fly term in Yahgan), with a hint of relative dominance implied as well. While English has a plethora of colorful expressions for denoting such circumstances, Yahgan has reduced the system to a well-defined smallish set of terms from the domain of posture verbs. Such reduction is one symptom of grammaticalization.