For Intermediate players, "Reversing" is possibly the least-understood area of the game. The topic is so confusing that I sometimes hear people ask: "Do you play reverses?" or "Should I play reverses?" This isn't something which is optional. It is part of the bidding, like it or not.
After opening 1 of a suit, we have to examine the situation where the opener's rebid is on the 2-level (or higher) in a higher ranking suit.
Here are some examples:
A) 1♦ P 1♠ P 2♥
B) 1♥ P 1NT P 2♠
C) 1♣ P 1♠ P 2♦
In all cases, opener has "reversed." Doing so shows a good hand, at least 16 or 17 (depending on how you count distribution). In the examples above, opener might have:
A) ♠K32 ♥AQ32 ♦AKJ65 ♣2
B) ♠AQ65 ♥KQJ76 ♦AQ2 ♣3
C) ♠-- ♥AQ3 ♦AKQ2 ♣K108765
Reversing typically shows 4 cards in the suit reversed into and at least 5 cards in the suit opened. As stated above, it shows a very good hand. With a so-so hand (such a 12, 13, 14, 15, 16 points), opener must not reverse! He has to find some other rebid. In the following quiz, you can test yourself.
Note: I do not recommend getting involved with this issue after a 2-level response. I want everyone to play that a 2-level response is natural and Game-Forcing. After this, opener can "reverse" without promising extras. So, 1♥-Pass-2♣-Pass-2♠ while a "reverse," should not guarantee (though it may contain) extras.