When your partner opens 1nt and you hold a decent hand with 5/4 in the majors Smolen is a very useful convention. You bid 2C asking for a major and if your partner responds 2D you jump to 3 of the major in which you hold 4 cards. This way your partner knows to bid the other major. If he denies 3-card support by bidding 3nt and you hold 6 cards in that suit you will correct to 4H or 4S. This is a variation of the transfer bid that we do directly over 1nt.

Mini maxi is another treatment when your partner opens 1NT. Holding 5/5 in the majors you show the strength of your hand by your bid. Most play that 3H is weak and 3S is strong and game forcing. This is one use of the 3H and 3S bid over 1NT and is not standard but some partnerships play it with success. Some also play that 3C and 3D do essentially the same thing, however this is not my preference.

Hand 1

Partner	KJT54 AJ96 Q6 54

	AQ9 Q75 AT4 AJT9

North	East	South	West
-	Pass	1NT	Pass
2C*	Pass	2D*	Pass
3H*	Pass	3S	Pass
4S	Pass	Pass	Pass

South opens a strong 1nt. North has a very nice hand and bids stayman. South then bids 2D denying a 4-card major. With 5/4 in the majors and a Game Forcing hand North then Jumps to 3 of the suit which he has 4 cards in, thus showing a 5-card Spade Suit.
With 3 card support for Spades South bids this Major and North signs off in 4S.

Hand 2

Partner JT98
KT764
Q5
54

KQ7
QJ6
KQT3
AQJ

North

East

South

West

-

Pass

2NT

Pass

3C*

Pass

3D

Pass

3S

Pass

4H

Pass

Pass

Pass

-

-

When South opens a 2nt, North has a very nice bid in his arsenal.

Smolen can also be used over 2nt. North bids 3C (Stayman) looking for a 4-card major and when South bids 3D denying a 4-card major, North will bid 3S alerting his partner that he has a 5+ Card Heart suit. With 3-Card Support for Hearts South bids 4H.

Not all partnerships bid Smolen after an 2nt opening -- this is something that you should discuss with your partner.

References ACBL Website