When you press down on the flush handle, its lever lifts a chain or lift wire attached to a flapper or tank ball. (Flappers have chains; tank balls have lift wires.) The chain or wire lifts the flapper or ball from its flush-valve seat, which allows water to escape through the valve.
Both flappers and tank balls are hollow. Some are open at the bottom, and some are closed, but both drain water and trap air, making them temporarily buoyant when you push down on the handle. Without this buoyancy, the flapper or ball would sink immediately, so you'd need to hold the flush lever down until the tank was empty.
As the flushing water recedes, the flapper or ball floats downward until it rests in the flush-valve seat and stops up the opening.
After passing through the flush valve, much of the water shoots through the siphon jet, the small opening across from the trap outlet. The rest spills through slanted holes in the rim.
The water rushing in through the siphon jet overflows the trap, priming it. Once primed, the trap siphons all the water it can over its weir, or crown, stopping only when there's not enough water in the bowl to sustain the siphon. At this point, all water on the house side of the trap slides back into the bowl.
Because the rim holes are slanted, the water entering the bowl through the rim travels diagonally around the bowl. This diagonal pattern scours the sides of the bowl, but it also sends the water over the trap in a spiraling motion, which improves the efficiency of the flush.
As the bowl empties, new water enters the tank through the ballcock or fill valve. This flow begins the moment you flush the toilet. Most of the water entering the tank does so through a tube that terminates near the bottom of the tank. Delivering the water to the bottom of the tank provides a measure of noise control. The sound is muffled as soon as the water in the tank rises above the end of the tube. At the same time, a small steam of water is diverted into the flush valve overflow tube-via a 1/8-inch-diameter fill tube-and falls directly into the bowl's rim. This steam restores the water in the bowl to its maximum level. As soon as the tank fills, the float shuts off the fill valve, and the toilet is ready for another flush.