Negative Slam Doubles

After a competitive auction where the opponents land in a slam and you and partner have raised a suit preemptively, a double by the immediate hand after the slam bid shows 0 defensive tricks. Partner
leaves it in with 2 tricks or sacrifices with 0 or 1 trick.

With 1 or more tricks, the immediate hand after the slam bidder passes. With 0 tricks, partner doubles which the first partner can leave in with enough setting tricks OR take out for a sacrifice. 
With 1 or more tricks, partner passes. 

Why should partner pass? Well, if your partner does have 1 trick and you have 1 trick then the opponents are down in a slam-- which should be a good score in matchpoints since most of the other players will be making their game contracts.