In this diagram, Blue has placed a post hoping to prevent Red from connecting the fence on the right to the fence on the left. If Red tries to go below as shown, Blue will connect to the bottom, blocking Red. Likewise if Red tries to go above the blue post. But there is a way for Red to get around Blue.
If Red extends the fence on the left, then as shown by the hashed posts and fences, there are two ways for Red to connect the two fences.
If Blue plays to block the lower connection, Red can connect above.
If Blue plays to block the upper connection, Red can connect below.
So after Red’s move, Blue cannot prevent Red from connecting the two fences. The points to the left and right of the blue post will fail for the same reason: since all three lie within the diamond shape created by the two possible connections for Red, a blue post on any of the three points can connect to block either up or down, but not both, leaving Red to take the other path.
This connection is called the Long Diamond because of the shape the two possible paths make. The long diamond allows you to jump four points up, down, left, or right, even though your opponent has posts on any (or all) of the three intervening points. The long diamond works only because there is no way for your opponent to block both paths in one move. If your opponent has any posts just outside the long diamond, he can block both paths in one move and the long diamond will fail.
Note that Red has no control over whether Blue blocks above or below. Blocking above, heading out toward the rest of the board, may be beneficial depending on the configuration of the rest of the board. Blocking below has some immediate effects that are interesting.
In this board, Blue blocked below and Red connected above. Blue then formed territory, which leaves Red with no territory at the moment. Red can put in fences on the left and right, but he will likely end up with 9 units of territory or less, where he was hoping for 21.
In exchange for this reduction of Red, Blue has given up any hope of connecting to the outside. His territory will be worth a few points at most.