Snap Dragon Doubles
After a competitive auction where the LHO opponent opens, partner overcalls and RHO responds a new suit, a double would show the 4th suit (at least 5) and support for partner (usually 3).
Of course, the snap dragon doubler is not expected to have many hcp.
Examples:
opp part opp you
1H 1S 2C Double = Showing diamonds and
spade support
or
1S 2C 2D Double = Showing hearts and club
support
or
1S 2C 2D 2H DENIES club support (else snap
dragon)
or
1S 2C 2D 3C tends to deny having hearts