Product waitlist feature

Hi,

Could someone explain how waitlist feature does work?

By default It looks like that user gets subscribed to default product variant.
As example if we have dress with sizes S M L and default is set S in back-end, product sold out and it will be subscribed to dress S size....which is set default in back-end. So it means that if we add stock to M variant user wont get email? Or system checks main product quantity?

We are thinking to add waitlist button directly to product page even there is some quantity and add to cart button. Make it something like "Can't get a color or size you want? Join the waitlist".

It works fine and you can subscribe to waitlist but not sure about emails. Will user receive email if main product quantity was 0, user subscribed to waitlist, we modified main product quantity to 2 ?

Kind Regards