Why does water syphoning work, ie why does the water initially flow uphill? I need a very detailed description

The water must be given as initial push to go uphill. Once the flow is established, siphoning can occur.

The water does not INITIALLY flow uphill; it has to be given a start. For demonstration purposes, this is sometimes done by creating a suction (using your mouth)at the outlet end and immediately ensuring that that end is lower than the inlet end in the reservoir. The suction creates a partial vacuum resulting in atmospheric pressure at the surface of the reservoir being greater. Consequently, that greater atmospheric pressure pushes the water up. If you try to perform syphoning from a totally sealed container with only the hose going in, it will not work.

The outlet end must be kept lower than the inlet. Gravity acts on both sides but since the outlet end has the greater column of water, the stronger force is exerted on this side, thus withdrawing it. This continues the initial partial vacuum, and atmospheric pressure continues to push the water up the hose.

In summary, two factors work together: gravity and atmospheric pressure.

You first need to know about potential energy:

Potential Energy:

Potential energy exists whenever an object which has mass has a position within a force field. The most everyday example of this is the position of objects in the earth's gravitational field.

The potential energy of an object in this case is given by the relation: PE = mgh

where

PE = Energy (in Joules)

m = mass (in kilograms)

g = gravitational acceleration of the earth (9.8 m/sec2)

h = height above earth's surface (in meters)

as long as the potential energy of the water at the end is lower, siphoning will work. see webpage