After making that "soup" of flavour off the bottom of the roasting pan, after adding hot water, we pour that mess thru this gizmo, the grease floats to top, pour the good stuff out of the spout, comes from below the grease. Stop pouring when grease starts flowing down spout .
Make sure the gravy boils to cook the flour slurry, or the gravy may taste like "paste".
Cheers