The hips provide the power and leverage for all koshi waza or hip throws. By placing your hips lower than you opponents center of gravity and using your hips as the fulcrum of a long lever (the lever being the length of your body plus the length of your opponents body) you can force the opponent to tumble over you and crash to the floor! Hip throws are among the hardest impact throws in Judo.
Most but not all koshi waza are executed in a forward direction – the direction in which your opponent is advancing. There are many effective ways to perform these techniques under a wide array of circumstance: including non standard grips, counterattacks, and combinations. A great hip throw can end the fight quick!
Check out this great old school footage of a Koshi Waza demonstration:
An example of koshi waza in competition: