Over 35 years ago the Austrailian government asked Caroma to help them achieve much needed water conservation goals with more efficient toilets. At the time over 35% of the water used in a home goes down the toilet! After many years of reseach, Caroma had invented the first Dual Flush toilet. The optimum toilet to conserve water and without compromising performance.
Siphonic vs. Washdown
Siphonic toilets swirl water around the bowl to create a vacuum or "siphon" in the trapway so water leaves the bowl first, pulling waste out afterwards. While this system works very well when large quantities of water are allowed, it is much more problematic when water is limited to 6 liters or less. To still be able to create the Siphonic action, trapways had to be reduced to around 2 inches. Naturally, the reduced trap size made clogging common.
Wash down systems do not have to create a siphon in the trapway. A steeper pitched bowl coupled with an open rim design pushes water very quickly through the trapway, forcing the waste out first, followed by water. There are several advantages to wash down. Trapways are able to stay much larger. CAROMA toilets feature 4-inch trapways that are almost impossible to clog. Caroma toilets also allow for superior line carry, as waste is forced out first, followed by the water.
Now here's a video: Caroma Washdown vs. Siphon flush Technology