If the Amazon product description doesn't make it clear, you put a round or hex thing through one of the two holes in the plate and then tilt the plate, bringing the edges of the hole up against the side of the thing, jamming it in place. I am not familiar with InstaVise specifically, but there are lots of these
"floor vises" or "plate vises". Often they are intended to be held down with one foot maintaining the jamming action while you use a saw to cut the pipe/conduit/UniStrut/whatever. The InstaVise looks like it's too small for that, so that boss with the hole probably attachs to some other part of the device.
Look up TriVise and LumberLok for similar plate vises.