Right hand opponent (RHO) opens 1D Sitting South, you'd like to be able to make a bid holding

      S. A 10 8 5 2      H. J 8      D. 6      C. K Q 7 5 4

You have a nice two-suiter in a 6-loser hand which you'd like partner to know about. If your two suits were spades and hearts, you could use Michaels here, promising 5-5 in the majors. Also, if your two suits were hearts and clubs, you could use the unusual no-trump, promising the lower two unbid suits, hearts and clubs.

Unfortunately, your two suits don't fit either of these handy conventions, so what to do? In fact, you can still use the Unusual NT here, provided you and partner agree to allow overcaller to "correct" advancer's choice.

Here's how it would work with the hand shown. After the 1D opener, you overcall 2NT. Say your LHO passes, and your partner (advancer) chooses 3H. as he most likely will. Assume your RHO passes, and this comes round to you. Now you correct 3H. to 3S. , which says to partner: "I actually didn't have hearts and clubs as I was supposed to have, I had spades and clubs." When 3S. comes back to partner sitting North, she can either pass or correct to 4C. , and you have therefore found your best fit.

Do be aware that this extension to the unusual NT can push you a level higher, since it might be the four level (as here) before you find your best suit. Since there is, therefore, some risk in using the extension you'll have to have better hands for the unusual NT than pairs who do not play extended unusual NT. Doubtless, this is why the extension isn't popular.

There is a specific convention for handling the particular hand above, its called "top and bottom cue bids". Alas, this gadget precludes usuing Michaels cue bids, which is why "top and bottom" has never become popular.

All said, you don't get something for nothing!