Using 2- and 3-under as 2C Responder

Another possible use for the two-under or three-under (for transfers) might be in responding to a strong 2C opening.  Suppose that 2D is waiting and GF, 2H bust.  That leaves 2S, 2NT, and the three-level for positives, and possibly for "preempt-ish" hands.  Stacking is facilitated by some two-under and three-under techniques.

Two Spades as One or Both Minors

Here, a two-under approach is used, with a Smolen style of follow-ups.

Opener generally will bid 2NT if he wants to agree diamonds or Three Clubs to agree clubs, assuming initially a two-suiter.  After 2NT (prefers diamonds), Responder can use a "Smolen" style to unwind his hand, bidding 3C as a "transfer" to agree diamonds, 3D to suggest clubs, or 3M as shortness.

Two No Trump as One or Both MAJORS

With a one-suiter and a major, Responder uses a three-under, enabling transfers.  Opener assumes a two-suiter and picks his major of preference, bidding 3H to suggest hearts or 3C to suggest spades.  If Responder has both, he can bid 4C or 4D as shortness.  Or, to simply agree spades (possibly one-suited), he can transfer with 3H after 3C.  3D after Opener's 3C shows just hearts. 

If Opener shows heart preference (3H) and Responder has just spades, Responder can bid 3S in a simple version, which is no worse than the usual method where Responder bids his own suit.

If Opener wants to reject both majors and show a long minor, he bids 3D with diamonds or 3S with clubs.  If Opener has his own spade suit, and cares not if Responder has hearts, he bids 3C and then insists with 3S after a transfer.  If Opener has his own heart suit and cares not about a one-suited spade suit from partner, Opener can bid 3H and then after 3S bid something above 3NT.

Major-Minor Combo

With a major-minor combo, Responder bids 3C.  Opener usually shows which major he would prefer, bidding 3D to preference spades.  If Opener wants to agree both majors, he bids 3S.

If Opener bids 3H and Responder has hearts and a minor, he can bid the minor next as a slam move.  If he has spades and a minor, 3S does the trick.  Opener can then agree spades with 4C to ask for the minor.

If Opener bids 3S, Responder can bid up-the-line.  3NT = clubs and hearts; 4C = diamonds and hearts.  4D = clubs and spades; 4H = diamonds and spades extras; 4S = diamonds and spades minimum.

If Opener bids 3D to preference spades, Responder bids 3H with hearts and clubs, 3S with hearts and diamonds extras, 3NT with hearts and diamonds minimum, 4C with clubs and spades, 4D with diamonds and spades.

Other Three-Level Calls

This allows 3D, 3H, 3S, or 3NT to show other holdings, whether 4-4-4-1 hands or perhaps one-suited but weaker.  Thus, for instance, 2C-P-3S could be played as 1-4-4-4 or could be played as seven spades but a bust.  Something like that.