Water doesnt flow from low pressure to high pressure, it flows from high pressure to low pressure. This is not exactly what is going on in a siphon tho.
A siphon uses gravitational potential energy to move the water.
Use Bernoulli's equation to understand the siphon.
http://en.wikipedia.org/wiki/Siphon use that to see the math and some very nice pictures. scroll down to bernoulli's equation..