The Unusual No Trump is a commonly met convention. When the bidder can actually have the values for a NT overcall, it is assumed to be natural but, when they cannot have such values, it is assumed to show a hand which is at least 5- 5 in the minors. Because it is in regular use, you must know what you are doing over it. In today’s hand, West would overcall 1NT with 15- 18 flat and double then bid NT with a stronger hand. Because of this, the jump to 2NT is not required in a natural sense and so it is “unusual”. When the 2NT overcall occurs, North has a problem. The North hand has only six losers and a bid of 3H shows eight losers and one of 4H shows seven losers and, typically, less high cards than this. Clearly, North should not be bidding any number of hearts here but what should be bid? There are a number of conventions of varying complexity which are on offer to handle this situation.
The simplest is to note that West has, in effect, bid both minors and that there are two cue raises available. One can differentiate the two cue raises by strength or by length of trump support. Another alternative is to use the jumps to four of a minor as splinter bids with heart support and values beyond those needed for a direct heart bid. That would certainly work well here as South would remove the two diamond losers from his assessment and see a four loser hand after hearing the 4D splinter. Since the splinter bid promised more than a raise to 4H it shows less than seven losers. South will have no difficulty bidding 6H over the 5D bid or even a more adventurous 6D, although double might be safer at teams. The most complex method is “unusual over unusual” where 3C would show a good heart raise and 3H a merely competitive one. Similarly, 3D would show a forcing spade bid while 3S would be a long, competitive spade holding. If you play competitively, you need to adopt some of the methods discussed to avoid unusual problems.