Hello there,
Good Day !!!
Sorry for lacking documentation over that option(we will sure add it in our plugin description page).
Actually we use WordPress Background Process Engine as the default(which we loaded background process library in core) so the functionality is same in both option means sending mail in a background process(queue concept). Only difference is you can track the scheduled background list of actions in WooCommerce Background Process Engine(Action Scheduler) also you can track the details if it is failed/executed in WooCommerce -> Status -> Scheduled Actions Tab
If you encounter any issue in Default try to switch WooCommerce Background Process Engine to see it works.
Let us know
Thank you.