A knock down golf shot is a shot that flies low.
It is a great shot to know how to play if conditions are windy as it will allow you to control your trajectory and hit the ball low under the wind for better accuracy. It is also a great shot to play from trees, when again you need to control your trajectory and keep the ball low.
To hit a knock down golf shot, set up with the ball towards the back of your stance - right of centre for a right handed golfer. This will encourage you to connect on your downswing before the true loft of the club face is presented to the ball, producing a lower ball flight. Angle the club shaft so that the handle of the club leans towards the target more than the club head does. This will help to de-loft the club face and produce a low trajectory.
The key to executing this shot well is to keep your hands forward of the club head, or to keep the lean in the shaft of the club as you connect. Only swing back to waist height and keep a really straight line from your left arm into the club, you do not hinge your wrist on this shot. Now drive the forward leaning shaft angle through the ball and finish in a shorter waist height follow through. Your follow through will be much more restricted than normal if you maintain the correct shaft angle through impact.
If this is a new shot for you, try making a few swings holding an alignment pole behind your left arm. Hold both the pole and the club and as you swing back keep the pole in contact with your left ribs. As you swing through, maintain the same contact on your left ribs. The use of the alignment pole will ensure that you maintain a really straight left arm and club position during the swing and will also ensure that you lead through impact with your hands ahead of the club head, producing a low flying knocked down golf shot.