Trying to get my stroke straight I put my thumb on top of the butt in line with the tip. I started to make balls as fast as I could put them on the table. It seems with thumb on top everything seem to line up,I'm on auto. I know this sounds silly, but please try this for one rack an reply. Thanks, P.S. Were is Efern?
I think that using the grip hand thumb-on-top method will work fine if the conditions are right.
If you have a relatively short bridge length (from CB to bridge hand), and you have a short stroke, then the thumb won't affect the shot. I am double jointed, so I can bend my fingers twice as much as a normal person. I have tried this method for more than 1 rack.
The problem becomes noticeable when you make longer strokes because the thumb will have to bend. The farther your back swing, the more stress will be applied to your thumb. So, if you are bunt stroking on a 7 foot bar table with fast cloth. Then, running racks with the thumb method is not surprising.
But if you need to get some power in your shots, then you need to have a longer stroke, and the thumb method will cause you to miss shots because you will not be able to strike the CB accurately/consistently.
Like another poster mentioned, if you are rest your entire thumb on the cue, then it is an ill advised method. But if you are just using the tip of your thumb, then it may not be a problem.
To add: I think you will find that the cue is more prone to slip out of your grip when using the thumb on top method. After a few embarassing incidents during match play, you may decide to go back to a full grip.
Try doing this.....line up your shot, and before you pull the trigger, bring your thumb down (like a normal grip). I think you'll find that the thumb is not really doing much. The wrist alignment is the heart of the matter.