It is my impression that what Boras does is hypnotize owners into making ridiculously high bids for free agents - remember, at one point he had Tom Hicks, otherwise a shrewd and ruthless businessman, bidding against himself in the A-Rod sweepstakes.
How he hypnotizes them is open to conjecture, I suppose. Good salesmanship? Witchcraft? Communion with (you name the dark force)? Personally, I believe he is one of those lizard people; you know, the ones where if you look away and then look back real quick, for an instant you see their lizard face instead of the human facade.
It is what Boras does next that is wickedly brilliant. Instead of coercing his client directly, he lets the union influence his client into signing the most lucrative deal, whether the player prefers to or not. For the good of all, you know. A rising tide raises all boats, etc.
This is all perfectly legal, and many would say just good bargaining. Many reservations about Boras seem to be on moral grounds, but of course morals have nothing to do with it.
If nothing else, Boras has been one of the best bogeymen in sports in the last twenty years. I'll bet Hicks still has bad dreams about him.