When it's on I cover the hole in the lower left corner and dip the yellow tip things into the trough of seeds. One seed is held onto the end of each tip. When I remove my thumb from the hole the seeds fall off in magically straight rows of 12 seeds. I do this twice to have 24 seeds per row.
This is especially useful when seeding lettuce because the seeds are so small. See the nice, not-so-densely seeded lettuce below? It's just beautiful.