Allows you to use the Ziploc manual vacuum pump on a Foodsaver jar, when you don't feel like plugging in the machine.
Large-face prints toward print bed so it's nice and smooth to make a good seal against the vacuum pump gasket. I printed with 2-3 extra shells as a safety against air leaks.
Apply a gasket/putty to the small opening (silly putty works up to a point) to give a good seal against the jar pump opening.
(Since silly putty is very malleable, it'll deform when there's enough of a pressure differential, at which point you've probably spent enough time pumping anyways.)
(The foodsaver jar lids don't seem to be manufactured with the same tolerances, so this may be snug or loose. Gasket/putty helps if it's loose; if snug, sand the adaptor down or if lazy, force the issue...)