how does HGV know what resort to pull from?
No doubt the terms and conditions allow the developer to estimate supply and demand in advance to allocate availability to the various internal and external exchange systems. I do include HGVC as an internal exchange system in this thought. HGV will not know with certainty how many owners of the various resorts will choose to book other HGVC resorts, bHC resorts, MAX resorts, etc., until those choices are made but does have to make some availability to the external ones (RCI, SFX, etc.) so that their contractual obligations are met and the external exchanges can be made. The contractual obligations can no doubt be at least partially satisfied with developer owned inventory, but there isn't a requirement to book a week and deposit it in RCI so why would there be a one for one identification of point sources needed for Max?
Isn't what is available in HGV Max based on what a member owns? If an owner owns HGVC Kihei and uses their points in Max instead, their Kihei week should now be available in Max. It isn't necessarily leftovers that show up in Max, it is what people deposit in there to book something else.
What if the HGVC Kihei owner uses 1/10th of their points in Max? That could be a possibility given the discounting we've seen on the HVC resorts in Max (up to 75% off) and the comparative point values for the weeks. I don't believe it would be a possibility for HGV to make 1/10th of a week available to Max - that would be 16.8 hours. Instead, I believe that it works very much like Marriott estimates for Abound/II/Trust usage.
I suspect they then take control of the good week and rent it for cash?
HGVC I am not sure but RCI yes...
If one can prove that rental of good weeks exchanged happens, one could seek compensation from RCI. RCI does have different pools of exchange availability for the various corporate account systems, RCI Select, and The Registry Collection, so the good weeks might be going there or to OGS rather than to rental. I'm not defending any practices that Wyndham has with RCI because it's pretty much a black box to me, similar to the HGV allocation process for exchanges, but very likely has internal controls and does have a separate corporate entity (Resort Rental, LLC) that does the rental side of things. I know that for some resorts there are separate resort IDs for their rental programs (e.g., Buganvilias). There might also be provisions allowing use of exchanged weeks after a certain period of availability for rent as getaways and certainly is for last calls.