What causes a brick to blow away perpendiculars?

Question

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?

Answer 1

If you do a 4 block match horizontally or vertically, you make a bomb. If you make a T or L shape of one color, you make a lightning piece that acts as you noted. It is useful when you're running out of options, to clear part of the screen away, to get more pieces--it's also useful in creating combos at times from falling pieces.

T shape:  
X X X  
  X
  X

L shape:  
X X X  
X
X