A hook or slice is created when the ball is given a spin on its horizontal axis. The ball will be deflected to the right for a clockwise rotation and the left for a counter-clockwise rotation. This generation of an aerodynamic force is known as the Magnus Effect.
The Perfect Practice Tee helps to identify your swing motion by examining the tee angle after your follow thru. If the tee is centered in the 90 degree cut out you know you have Straight on Driver Contact to the ball therefore minimizing rotation forces.
If the tee angle is laying slightly to or away from you in the 90 degree cut out this identifies that you are not hitting the ball straight and you are introducing rotation forces on the ball that need to be corrected.
For a severe slice, try aligning your body slightly to the right and closing your stance to encourage your swing to follow an inside out path.
By opening your stance and aiming your body slightly to the left, your body position will allow you to make an outside-in swing and should help to straighten out a severe hook and get your ball back in the fairway.
One last thing
--If you work an inside out swing or use a slice to your advantage--perfect.
The way the tee lays in the 90 degree cut out shows that you are working your shot perfectly-- and doing it consistently over and over. It doesn't have to be in the middle to make it right just so you will know that what you are doing, you are doing it consistently!