He disguises it extremely well and most importantly, bowls it at just the right pace. I have noticed that Lee's slower ball is usually really, really slow and thus not effective at all.
Had Hoggart been bowled on that full toss (which he ducked) it would have been the most hilarious dismissal of the entire series. The ball missed the stumps by a few inches, I think.
Nowdays the bowlers roll their fingers over the ball which is a perfect disguise to ball the slower ball and almost undetectable.
I remember in the old days, you had to use the split finger grip which was easily detectable.
Aaqib was the bolwer who used the slower bowl to great effect, still remember his wicket of Greatbatch in the semi final WC 92 with the slower ball. I dont know about others but Aaqib bowled that with finger rolling over, off cutters and Greatbatch just helplessly saw the ball going into the stump after he had come down the track.
The slower ball Shoaib balls is a big threat since he regularly bowls in 90 mph range. Many bowler can throw in a 90 mph ball but not many can do it regularly over after over. And with his long run up and contant quick speed the batsmen are always looking for a fast ball and when he throws in a slower one it's really hard to pick it up. Plus Shoaib's slower one is kinds loopy so you dont really know it ift's a low full toss or a yorker or a slower one.