How does a steam dishwasher work?
There is no small compartment. From what I've observed, the heating element kicks in and the water in the tub is raised to a steaming hot level. For the "Sanitize" cycle, that means 174 degrees. For the "Heavy" and "Full" steam cycles, it's somewhat lower.
Wash action stops while the water is heated. You can hear the fizz of the bubbles hitting the surface when the water is up to steaming temperature.
This machine uses its share of water and electricity, but it's all business and gets the job done. For everyday loads I use the "Regular" non-steam wash cycle, which requires less use of the heating element and completes the work quickly. The steam cycles come in handy for baked-on bits of food. Even Thermador, however, admits that the machine is not a miracle worker and states clearly in the owner's manual that it won't remove anything that's burnt-on. I'm willing to cut it some slack on that point. It's a real pleasure to unload a top rack that's thoroughly dry (and it accomplishes this without any forced air), so I don't mind washing by hand any baking dishes or roasting pans that have burnt-on residue in exchange for that rare luxury.