There are different kinds of pins - some are "relative", and some are "absolute". If you have an absolute pin, that is something that looks like this:
No matter how much it wants to, the black knight on c6 cannot move. Even if the knight were in position to take a queen, it could not, because moving the knight would be illegal, since it would leave the king in check to the bishop on b5.
A relative pin is different. It might look something like this:
Here, the knight on f6 is pinned in a "relative pin". The knight is allowed to move, according to the rules of the game; but at a cost of the queen. However, a player should keep in mind that, for a big enough reason, the knight could still move.
A pin can be a powerful weapon, but it is easy to forget that if the pinning piece is unguarded, sometimes the pin can be turned upside down! This is the basis of Legal's Mate, a famous opening trap that your students should see. Not only is it important for a chess player to be familiar with this trap for its own worth, but also the very concept of a discovered attack on a pinning piece is important to be aware of. By the way, once during a tournament, as I was just starting my own game, I was shocked to look over at the board next to me and see that it had already ended...with Legal's Mate!
This is a famous game which was played in Paris in 1750. The player who had white was Kermeur de Legal - that's where the name of the pattern came from. It should be simple to show this game to the student (or students). Of course, keep in mind that you should always let them try to find White's moves.
This concept is the basis for many combinations, such as the following:
In both of these examples, the losing player relied too much on the power of the pin to protect them. But since the pin was only relative, the winning side was able to sacrifice the queen in order to break the pin. This example can also be used to illustrate this concept.
So remember: a pin can be powerful, but you cannot rely on it blindly - especially if it is only a relative pin. Occasionally (as Saint Brie found out) it can be turned upside down, and the pin can suddenly become a queen sacrifice leading to a beautiful checkmate!