I must have bought an older version of this bath tub (we bought it on Amazon Jan 2011), because the one I have is dark blue and not as sleek looking as the one in the picture. So my review will be of the older tub, but the concept is the same. Water runs into a reservoir at one end of the tub, where a thermometer shows you the temperature of the water. The overflow from the reservoir runs into the tub itself, and the overflow from the tub runs out into the drain. This way the baby gets mostly fresh and clean water in his bath. If your baby pee's into the tub (and believe me, this happens all the time), you just pull the plug in the tub and the water drains quickly, and then gets refilled from the reservoir. I like how the baby gets to bathe in clean water and I can always keep an eye on the temperature of the water. It's a great little tub. I did find though that it was hard to hold up a floppy newborn in the tub, and they seem to have fixed this by introducing the new cushion I see in the picture.
The description says that the tub fits in the sink but there was no way our tub would fit in any sink in our house, maybe except for the kitchen sink. It was way too big. We put it in the bath tub instead.