When learning the game you were probably taught that to overcall the opponents suit showed partner that you held a massive hand that could probably make game without any help.  Easy, but when did you last have such a hand?  Redundant bids need to be put to good use and many club players now use a cue bid of the opponent's suit to show highly distributional two suited hands.

After an opponent opens, it's often difficult to describe a hand with two long suits. The auction may get too high (or end too soon) for you to show both, or you may not have the strength to safely bid either suit.
To show two suits with one bid is an attractive proposition
Consider these problems if RHO opens:

(A)  RHO opens 1 or 1 and you hold:  KQ964  KQ872  52   7

(B)  RHO opens 1 or 1 and you hold:  765   -   K10765   QJ1093

(C)  RHO opens 1 or 1 and you hold:  4   KJ872   AJ10754  5

With (A), you would overcall 1 and hope to bid hearts later. With (B) and (C), it may be dangerous to bid at all, especially at the 2-level. Both hands have good playing strength, but neither can be described with a standard overcall, which tends to show a one-suited hand with more high-card points.

The Michaels Cue Bid and Unusual No Trump are conventions that let you show two suits with one bid. The convention you use depends on which suit the opponent opens and which suits you have.

Michaels Cuebid.

• Over a minor-suit opening, a Michaels cue bid (1-2 or 1-2) shows at least 5-5 in both majors.

• Over a major-suit opening, a Michaels cue bid (1-2 or 1-2) shows at least 5-5 in the OTHER major and an unspecified minor.

Unusual 2NT

This convention is a direct jump to 2NT over an  opponent's opening bid (1 by RHO, 2NT by you) to show the two lower-ranking unbid suits. You should have at least 5 cards in each suit for this bid. One of your suits is always a minor, but the exact two you promise depend on the opening bid:

• Over a 1 opening, 2NT shows diamonds and hearts.

• Over a 1  opening, 2NT shows clubs and hearts.

• Over a 1 or 1 opening, 2NT shows both minors.

In the example hands above, Hand (A) is perfect for a Michaels Cuebid. With (B), an Unusual 2NT shows both suits without promising great high-card strength. And with (C), your overcall depends on which suit the opponent opened -- over 1, you would bid 2 Michaels to show the other major and a minor; over 1, you would use the Unusual 2NT to show the two lower unbid suits.

Strength requirements

The strength you promise for these overcalls is about 7+ pts., but suit quality and playing strength are more important than high-card points. Michaels Cue bids and Unusual No Trump overcalls are usually pre-emptive in nature -- they show good suits and distributional values, but don't necessarily promise the high-card strength of a regular overcall.
They can, however, be used with strong hands where you want to force partner to choose one of your suits.
Over an opponent's 1 opener, for example, you can bid 2 Michaels with  AKQ98  KQJ92  K5  7,
and then jump to game in the major partner bids.

Some players prefer to make a simple major-suit overcall with hands with intermediate strength (10-15 pts.). They use the Michaels cuebid only for hands that are weak (5-9 pts.) or very strong (good enough to jump to game once partner shows his preference).

In general, though, the best rule is to just use your judgment. Decide if you want to emphasize one suit (usually a major) with a simple overcall, or if you want to bring two suits into the picture immediately with a Michaels Cuebid or Unusual Notrump. For example:

• Over a 1 opener, it's probably best to overcall 1 with  8  AQ976  53  KQ1043. 
  You could use the Unusual 2NT, but this would force partner to bid at the 3-level, which may be too high.

• If there's great disparity in the strength of your two suits, it may be better to overcall one of them instead.
  Over 1 by RHO, if you have  KQJ98  A  J10654  43, a 1 overcall is a better description than a Michaels Cuebid.

Other uses

These conventions are most commonly used in direct seat over an opponent's opening bid, but can apply in other types of auctions, too. If an opening bid is passed around to you, you can balance with a Michaels Cuebid or Unusual 2NT. For example, after 1 by LHO-Pass-Pass, 2 by you would be Michaels, showing both majors.
After 1-Pass-Pass, a bid of 2NT is Unusual, showing both minors.

You can also play that any "Unusual" Notrump overcall (even if it's not a jump) is the Unusual Notrump.
For example, after 1H by LHO-Pass-2H, 2NT by you shows both minors (similarly, 1H-Pass-2H-3H is Michaels, showing spades and a minor).

No trump overcalls at higher levels also convey this meaning: 1 by LHO-Pass-3-3NT by you shows clubs and hearts.  1-Pass-44NT shows both minors. The higher the level, the stronger and more distributional your hand should be for your bid.

Use your judgment

Like all bids, these overcalls give information to both your partner and the opponents. If you don't buy the contract, your bid may work against you because it gives declarer an almost perfect picture of your distribution. For this reason, you should only use these bids when your hand meets all the requirements for suit length and playing strength.

Be careful if your high-card strength is outside your suits. With a hand like  K  A6  J9853  Q7632 , your suits are too weak for a 2NT overcall.

Vulnerability should also affect your decision. Since partner is often forced to bid at the 3-level (sometimes with only 2-card support), a vulnerable Michaels or Unusual 2NT should promise more playing strength--  AQJ97  KQ1082  42  4  or  Void  KQ1084  65  A98732 .

Responding bids

Remember that partner's Michaels Cuebid or 2NT overcall is artificial. If your RHO passes, you cannot pass. For all Unusual 2NT overcalls and most Michaels Cuebids, you'll know the exact two suits partner holds, so bid your longer one. You should almost always bid one of partner's suits; don't be tempted to suggest any other suit as trumps unless you have great length and strength in it.

The level of your bid depends on your strength and trump support. With a weak-to-intermediate hand, bid at the lowest level available. With a stronger hand, you can jump in one of partner's suits to invite game or jump directly to game.

If RHO makes an intervening bid, you should compete if you have some strength and support for one of partner's suits. Partner is promising 5-5 in his suits, so any 3-card holding is good support.

If you're not vulnerable and you have a weak hand with strong support, you may want to sacrifice.
Suppose LHO opens 1, partner overcalls 2 (spades and a minor), and you hold  J964  5  A872 J874 . You know the opponents can make at least 4, so if you're not vulnerable, you should sacrifice right away by jumping to 4. With your long trumps and singleton (and filler in whatever partner's minor is), a spade contract should make at least 8 or 9 (and maybe 10) tricks.

Finding partner's unknown suit

The one case where you won't know partner's exact two suits is when he makes a Michaels Cuebid over a 1 or 1 opening (showing the other major and an unspecified minor). Since you know his major, you can bid it if you have 3+-card (or 2-card) support. If you can't support his major but have at least 3-card length in both minors, you'll want to play in his minor suit. To ask partner which minor he holds -- and tell him that's your preferred trump suit -- bid 2NT. Partner will bid his long minor and you can then pass, raise or sacrifice.