It never books anybody, and that is the point
Plenty of systems promise to "fill cancellations automatically". Read that closely and it means a patient can find an appointment on their calendar that they did not agree to, made at whatever hour the cancellation happened.
Velvet Lead will not do that. The waiting list is a notification, not a decision. Everyone it suits hears about the slot, everyone can see the same free time in online booking, and the one who books it is the one who wanted it that morning. The clinic ends up with one appointment, one patient and one person who is definitely coming.