Source: May 2016 ACBL Bridge Bulletin 
The value of a bridge hand, like currency, often fluctuates with events. At the bridge table, those events are the progress of the auction. Let’s say you pickup:
Neither side vulnerable, partner opens 2 and RHO doubles. Is this still a meager 5-point hand?
The Q is a valuable filler, the spade support is excellent, the K is enhanced because the doubler will often hold the A, the club singleton is obviously useful, and the heart length also bodes well. Assign the doubler four hearts, and it is easy to visualize partner’s likely heart shortness. In effect, you have good news in every suit.
Jump to 4 and don’t be surprised to make it, even if you are doubled. For example, give partner:
If the A is onside, partner rates to lose one heart, one diamond, and one club – ruffing his other minor suit losers before drawing trump. Meanwhile, the opponents can make a minor-suit game. If they reach 5 or 5, continue to 5 for a cheap sacrifice. This may seem like a lot of bidding for a 5-point hand, but straw has been transformed into gold. This hand has become The Little Engine That Could.
Now, suppose you are dealt:
Your side is vulnerable, LHO opens 1, partner passes, and RHO bids a game-forcing 2. Does this qualify as a bidding problem? If your reaction is, “2 – what else?” then Houston, we do have a problem. You have 15 HCP and a moderate six-card major; how can it be wrong to bid 2, you may ask.
First of all, how much strength do you expect from partner? There are opening bids at your left and right, leaving partner with next to nothing. Second, are your honor cards well-placed? Their location could hardly be worse; the heart suit lies over your tenace, and your doubleton Q rates to be useless. Let’s say you find partner with a great buy under the circumstances:
If your side saves in 4 over 4 , expect to lose one spade, two hearts, one diamond, and two clubs, far too expensive. Perhaps you now concede that it is too dangerous to bid, but what about lead direction against 3NT? There, you might have a point. However, if partner is a fine player, with a hopeless hand he will try to find your length because you hold the defense’s entries. Expect a spade lead, likely partner’s shortest unbid suit, with fair frequency. If your 2 intervention runs into a trump stack on your left, how likely is it that 2 will be doubled, especially in this day and age of amorphous or DSI (Do Something Intelligent) doubles?
[box]Warning: After a 2/1 response, double by opener should be penalty.[/box]
There is no need for a support, general strength, or takeout double. Because the 2/1 bid guarantees that the auction will continue, both opener and responder can easily describe other features, such as three-card support or second suits, on subsequent rounds. But when opener holds length and strength in the overcalled suit, plus a misfit for responder, double is the ticket.
If responder has a reasonably balanced hand without a fit for opener, 2 doubled is going to be a bonanza for the doubler. Let’s say the overcaller’s dummy hits with something dismal:
Your hand, again:
The price tag for 2 doubled is probably 800 or 1100, and the opponents may not even have a game! On a different layout, if responder removes the double because of a spade void (say), the partnership is well positioned to aim at notrump. One more point in favor of the pass instead of overcalling: Declarer may place some missing high-card points with partner instead of marking them in your hand. What an interesting situation; so many points, yet so little point in showing them. This hand is an example of The Big Engine That Couldn’t.