What are sweep-in FDs, which lets people earn more & withdraw FDs midway too?

Sweep-in FDs link savings/current accounts to a fixed deposit facility that lets account holders earn the higher interest of FDs while keeping their funds comparatively more liquid than a traditional FD. Whenever funds in a savings account exceed a customer-decided threshold, the excess money gets moved into an FD. The sweep facility even allows partial withdrawals from the FDs.