So I'm trying to wrap my head around the physics of how a Soxhlet extractor works, not the chemical, solvent and chemical components part, but rather the physical, hydrodynamic, siphoning part : why exactly does the solvent flow back through the siphon tube into the flask ? I imagine Bernoulli's principle, comes into play, but, unlike a usual siphon tube, there's nothing or no one physically initiating the siphoning by sucking air out, or lowering the pressure in the flask. I've tried to find any theoretical study explaining the physics behind a Soxhlet extractor, but to no avail. Any help would be appreciated, thanks.
Soxhlet extractor, source: https://commons.wikimedia.org/wiki/File:Soxhlet_mechanism.gif