As you rightly point out the ball is not in play while it's sat on the tee, however, the ball is regarded as in play once you have swung the club with the intention of hitting it, otherwise known as a 'stroke'. For your convenience I've appended the definition of a stroke from R&A's rulebook:

A “stroke’’ is the forward movement of the club made with the intention of striking at and moving the ball, but if a player checks his downswing voluntarily before the clubhead reaches the ball he has not made a stroke.

So when we see Tiger checking his swing (stopping) as he does from time to time, this is not regarded as a 'stroke' as it was voluntarily, but, if he had missed the ball because something startled him, then that would count as a stroke.

A ball is “in play” as soon as the player has made a stroke on the teeing ground. It remains in play until it is holed, except when it is lost, out of bounds or lifted, or another ball has been substituted, whether or not the substitution is permitted; a ball so substituted becomes the ball in play.