Note: In competition only DBL maybe a check-back!!!

Non-forcing Check-back

This occurs after 1m-1M-1NT-2C

Forcing Check-back

This occurs after 1m-1M-1NT-2D and is the only way to make a game forcing bid.

Wolf Sign-off

This is a "negative ping-pong" occurring after:
Opener has to bid 3D after which responder bids:
By inference any responder bid other than 3C is strong.