Bergen Two-Under Transfer Preempts

Preemptive openings of 3C and higher can be used to show the suit two steps higher than the one bid. For example:

a) S x b) S x
H QJTxxxx H KQTxxxx
D xx D Kxxx
C xxx C x

These are possible 3H bids, but are substantially different in playing strength. If 3H is bid with both hands, the responder may have to guess. Bidding 3C to show a 3H opening allows the responder to bid 3D if he wishes to invite 4H. This has the usual advantage of transfer bids, in that partner becomes the declarer and the lead comes up to his possible tenace position. A disadvantage is that it places less pressure on the opponents in the bidding: Second hand will have two opportunities to act.
From The Official Encyclopedia of Bridge, Fifth edition