as soon as one’s back is turned

Would you tell me whether I am right with my interpretation of the expression in bold in the following sentence?

As soon as his back was turned the air felt lighter. (G. Butler, “The Way of All Flesh”)

as soon as one’s back is turned = behind someone’s back, in someone’s absence

Thanks for your efforts.

the moment he turned away…

as soon as - straight away/at once
his back was turned - he turned so they were viewing him from the back

In my poor opinion “as soon as” = “when” or “while” in the present case.

as soon as = (not later than the moment) when

In actual fact your interpretation in two of the expression in question tally with my one-act interpretation.