Waldeburg, In the DG site they went into more detail about construction tools, and potting medium. The tightly packed medium at the bottom acts as a wick to draw the water up to the higher levels and a cover helps slow evaporation from the top. As the plant grows, the roots will go down to the colander. A couple of things of note are type of medium. Needs to be fairly light so as not to crush the colandar and needs to wick the water up well, and NOT anything that contains fertilizer already as it won't get washed through as it would with top watering. Another point I learned the hard way, is to leave an inch or two of airspace above your drain tube for oxygen. I was going for maximum water capacity and didn't, so the medium soured somewhat and the tomatoes don't thrive as well as they should. third point is not to over fertilize! ( I did that last year too).
As to seedlings, if they are in the right temp and light, they should grow roots fast enough to be happy with the original soaking you gave the medium when you assembled it, especially if you have it covered. If you are sowing seed directly or the plants are tiny, you can lightly sprinkle from the top until they get going.
Hope that answers your questions...?
Carol
