How do lights with a dimmer switch work?

I have two 1000 watt HPS lights on a digital ballast. The ballast has a dimmer switch at 50%, 75%. and 100%. Can I assume that when I put my clones in that I can set it to 50% for a while so they get used to the lights. Does that really mean 500 watts? Then after a while I go to 75% and thats 750 Watts. I am trying to figure out if it safely translates this way.

I think 2000 watts is too much for my 8 x 4 x 7' tall tent. I was hoping to run it at 75% power in the long run and maybe push it to more if the plants can take it.

Yup,thats how they work. I love them. Mine are slightly different. They are Lumateks and they run at 600,750,1000 and overdrive which I think is like 1100 (but that supposedly shortens the life of the bulb). I have a bunch of them and in summer months I run at 750 most of the time to keep things cooler.