A syphonic valve begins the flow of water from the cistern into the bowl when you press the flush handle by lifting a column of water inside the syphon (the big central plastic bit in the cistern) which begins to fall down into the bowl. The rest of the water in the cistern is then pulled into the bowl by this initial column of water - just like syphoning petrol out of a car.
A flush valve simply opens a valve located at the bottom of the cistern and the water falls straight through the hole into the bowl.
The important issue for strength of flush is the volume of water - either valve just the job just as well, but the volume of water required to give a good flush is determined by the toilet pan design. Older toilets need 9.5 litres to give an effective flush, newer ones 6.5 litres. If you have an old toilet with a weak flush see if you can increase the water level in the cistern by adjusting the ball valve arm - making sure that the water level still remains below the level of the overflow when the cistern is full.
Durham Plumber, September 2005
