Here's how we use ours.
1. Swtich to wet setting and pump a couple of times so there is a little water
in which to do your job.
2. After job, pump on wet until all waste is well gone and bowl water is clear (or whatever color the water is at your location).
3. Pump on dry to remove water from bowl and get a little air in the line to break syphon (reduces bleed back)
4. Pump on wet a couple of times to get small amount of water in bowl. This completes the air lock in the line.
5. MOST IMPORTANT: Change valve back to dry. Most heads are at or below water line and you can sink the boat unless seals
are perfect and there is no such head
Hope this helps.
Of course, don't put anything in the head you didn't eat first. Toilet paper, used sparingly, is the one exception.