A pattern interrupt is an interruption in a specific flow or sequence. The main aim is to prevent any obvious linkage and thereby neurological conditioning. The other purpose is to weaken the old strategy and prevent it from having the same level of intensity as before.
If you knew that looking at the color blue would make your client feel fear, all you need to do is to interrupt the pattern before the fear kicks in. Display the stimulus, then interrupt the pattern. This can make the stimulus lose the original effect.