Rosenkranz Double

Rosenkranz Double is an artificial raise of overcaller's suit, showing an Ace or King in that suit.
By extension, a natural raise of the suit would deny an Ace or King in the suit.

This is a defensive bid, intended to make it easier for the overcaller to know how to play the suit on defense.

For example, if the overcaller knows that partner has a top honor, he can underlead his own top honor to put partner on lead. Underleading with a low card tells partner to lead back the lower side suit, while underleading with a higher card asks for the return of the higher side suit.

This can be a huge advantage if overcaller has a roughable void or an AQ suit he wants led through declarer, etc.