ThirdHome is an independent exchange system that allows you to establish an account based on an ownership somewhere in a timeshare that they will take deposits on. Once you've established an account, you book a week at your resort in a desirable season and deposit in ThirdHome in exchange for "keys", which are what they call the currency they use for exchanges. The exchange fees are on the high side, but get you access to some rather nice vacation homes and higher end TS/fractionals. There are some systems they do work with; in Marriott it's the Ritz Carlton and St Regis levels that are directly affiliated.
I mostly use my HGVC ownership (home week only as that's rentable) and WorldMark (also rentable) as they give me the most efficient return (number of keys per MF $). I have used them with some Vistana weeks as well, but the return isn't as good. Depending on what you own in HGVC and Wyndham you might be able to do fairly well; with Wyndham you have to be aware of the limitations on guest certificate usage, etc., of course.
When someone books the week you deposited, ThirdHome contacts you and you have to set up a guest certificate for the other member that booked your week. It's all fairly simple.
If you're interested, PM me and I can share my point of contact with them to discuss it with you. They would give me a referral credit, which is always nice; I think they give you one or two keys for setting up an account and making a deposit as well. Happy to answer any other questions you might have about them, too.