Snap Dragon Doubles

After pard has overcalled and RHO bids a 3rd suit, double says:

1. You have the unbid suit
2. You have tolerance for your suit (named in the overcall)

It can be quite useful, at least until someone comes up with a better usage for the bid. In terms of frequency of use (cost), it does not interfere with anything except those oppomnents who use negative free bids,e.g. that bid of 3rd suit not forcing (and you wish to make penalty double).
One has to have a hand commensurate with the level you are forcing partnership to rest. For example:
1C - 1H - 1 Sp - Dbl forces partnership to 2 level 1D - 1 Sp - 2H - Dbl forces partnership to 3 level etc.

Source:Harold Simon (from e-mail sent me on discuss@OkBridge.Com)