This particular drill can be used for two different purposes, one to help fix your slice, and the other to reduce your lower body movement (which occasionally can also lead to a slice). In regards to specifically fixing your slice, swinging from your knees will promote a flatter more baseball-like swing path into the ball. This should help you move away from a common cause of the slice, a steep, upright swing. By swinging from your knees, you make an upright swing impossible… and you force yourself to flatten your swing path in order to make contact with the ball. I’d recommend staying away from this drill on a range like the one photographed here – the dividers can cause problems. Try this on an open grass range and tee the ball up – use a long iron, or wood if you’re adventurous.
Another common cause for the slice is an over active lower body. By swinging from your knees you basically take your entire lower body out of the swing, and force your upper body to coil to compensate. Therefore, this drill is very effective at reducing a common slice cause, an over-active lower body. By eliminating your lower body from the swing, you are forced to rotate fully, making it very difficult to "get ahead of the ball". This rotation, is an important factor in helping your square the clubface – leading to straighter shots.
Give it a try!