It's just another one of those rules that very seldom gets strictly enforced; I think most officials work with the intent of the rule--to avoid delay--rather than the letter (or in this case, the number) of the law, counting off exactly 6 seconds and blowing a whistle. (When you think about it, 6 seconds seems a little arbitrary, doesn't it?)
I've never seen it called on the field, and I remember back to when the rule was changed from number of steps allowed to 6 seconds. Still, it's on the books, and it's fair game to enforce for anyone who wants to be a stickler.
