Drain jetting is something else you can do with your pressure washer.

Drain jetting is the act of cleaning out drains with pressurised water. Instead of having a lance or Flat surface cleaner fitted onto the end of your pressure washer's high pressure wire hose, you have a special drain cleaning head. You feed this drain cleaning head and its hose down the drain and let it clean out the drain.

From Dual Pumps you can buy a drain cleaning kit to fit onto your pressure washer.