This drill is intended to help you feel the positions of your golf swing, and also to test your ability to maintain control and to vary the speed and timing of your golf swing.
The drill is very simple. Set up with whatever club you intend to use, perhaps even your driver. Take the club back to the top of your backswing, down into impact, and extend to a full balanced finish in excruciatingly slow motion. For most people, the slower you go the more difficult the drill becomes.
The key is that despite your slow-motion, we are still intending to make solid contact with the ball, with a square club face, and have the ball go at our intended target. If this drill is done properly, the ball will travel only a very short distance, perhaps 20 or 30 yards with a driver. When I am working on my golf swing in practice, I normally will hit one or two slow-motion swings, then four or five normal swings, repeating the pattern for a few cycles.