I've noticed that most of the time, in Bejeweled Blitz, a 4-block match create a bomb which blows up the 8 positions around it. However, occasionally, similar (or perhaps the same?) 4-block matches create a brick which -- when matched -- "zaps" all the bricks perpendicular to it in both X and Y dimensions (causing a major chain reaction in the process).
What causes the latter effect? How can I use it to my advantage?