Cutting a Diamond
The long diamond and the square diamond can go around a post in their way, but they can be stopped if they are used incorrectly.
Trying to go beside a blocking post will fail. Here, Red plays at 1 thinking he will bypass Blue with the square diamond. Blue 2 cuts him off. Red should have played at A or B, grabbing more territory.
Note that playing at B is secure for Red — any attack inside by Blue will fail. Playing at A on the other hand invites Blue to play at 2 anyway, or perhaps at B. Either way, Blue will make territory or escape to the right.
Breaking The Chain
Stringing together diamond connections invites attack. Here Red has played 1, thinking to connect the two sides and claim a large territory.
Blue 2 cuts Red off two ways, and Red must choose which connection to make. Blue will be able to block the other, or continue the invasion and make territory inside.
Note that here only 2 will work. No other point has the potential to form fences across both diamonds. Specifically, any approach from above will fail.