You probably don't face the downhill bunker shot very often, which is one of the reasons it's so challenging. But you can master it with good technique.
First, align your shoulders parallel with the downslope. Your left shoulder should be as low as you can get it, and your right as high as possible. This will help you miss the lip of the bunker on your backswing.
Second, take a wide stance with all your weight on the inside of your left foot. Your right foot is only for balance. You can see here that my ball is so close to the lip, I have to plant my right foot outside the bunker. That's OK, as long as you can maintain your balance.
Third, open the clubface, and make a vertical backswing. Make sure you swing the club up with extra wrist action to be sure the club clears the back lip.
Finally, as you swing down keep the face open and low through impact so the divot comes under the ball. Don't be afraid to hit the sand hard.
It's tough to get a consistent divot on downhill bunker shots because of the awkward stance. A good drill is to draw a line in a practice bunker representing your ball position and make a series of swings so the middle of your divot takes out the line. Do this until you can repeat the result consistently.
Tom Watson is a Golf Digest Playing Editor and the golf professional emeritus at The Greenbrier, in White Sulphur Springs, W.Va.