Potential energy, pressure, the siphon effect and buoyancy are the principles of physics used by a toilet. The operation of a toilet is powered by the law of conservation of energy while its function and capacity is calculated with Bernoulli's equation.
When the back tank of a toilet fills up with water at an elevated level, it contains a certain amount of stored energy that is released when the flapper valve is opened to release its contents into the bowl. The resulting kinetic energy released increases the volume of water in the bowl and its pressure in relation to its specific gravity. The water that was in the bowl maintains its position because it is buoyant enough not to flow over the curve in the plumbing pipe. The additional volume of water fills the pipe and pushes it over the curve, starting a syphon that drains the water from the toilet.