The hip slide is one of the biggest cuplrits to inconsistent ball striking. The hip slide is basically when you transition into the downswing and slide your left hip too far forward causing the upper half to lag behind. When the upper half lags behind, you will tend to hit behind the ball causing 'thin' shots and 'fat' shots.
In a nutshell what should happen at the time of impact is that the left shoulder and the left hip should be in a straight line. The hip should also have cleared to the left with the rear pushing back. When the left hip performs it's function properly, the upper half will be more on top of the ball at impact causing the hands to push forward and thus hitting the ball first followed by the grass. This is the key for controlling your ball flight.
As mentioned in previous blogs, the correct function on the downswing is that the left hip should bump to the left approximately two to three inches followed by a rotation to the left. During this rotation the upper half should also come through the ball.
Now when you realise that you are sliding your hips too much, you can do a combination of two drills. The first one is where you would have your left foot back in line with the heel of your right foot. You would also place your feet fairly close together. The goal then is to swing the club and as you transition on the downswing to feel as though you are slightly bumping your left hip followed by a rotation to the left. This drill should give you a real sense of how your body should move through the ball.
The second drill is placing an alignment stick in your left hand and position the stick so that it's touching your left side. Use a SW and just ensure that when you hit the ball that the alignment stick doesn't contact your body. As long as the stick doesn't contact your body, it will mean that your upper half is coming through in sequence with your lower half and thus pushing your hands forward at impact. This drill is perhaps the best one I've tried as when placing the swing on the video with this drill, all my positions are hit perfectly.
Once you've practiced with these two drills, you want to then transition to a real swing with an 8 iron. The way I try and replicate this feeling is by imagining that I still have an alignment stick in my left hand and not wanting for it to hit my body through impact. I would then recommend placing your swing on the video every 10-12 shots until you see the perfect impact position.
If you have any further questions on how to prevent the left hip sliding, just contact myself for a lesson.