I had not thought of it at the time... and had not realized exactly how my head was plumbed... but when I first used it, it pumped sea water without me having to plug the sink first.
Later, when I realized what all the connections were, I started using fresh water flush instead... which ultimately lead to a much nicer smelling flush situation.

But indeed, I never had to plug the sink to pump sea water first. I would suppose it is due to the intake of the head being below that of the water line and below the level of water in that thru hull connection to the sink drain.

The fresh water flush setup really works well.