What does "不要紧" mean? 众人回头观望，而这一看不要紧，所有人都是不由大吃一惊


According to an online dictionary, "不要紧" means "not important". How does the sentence make sense?

"The group looked back and that look was not important. Everyone couldn't help but be shocked greatly?"
不要紧 has two meanings (http://www.baike.com/wiki/%E4%B8%8D%E8%A6%81%E7%B4%A7):

Doesn't matter; not a problem.

Seemingly not a problem but ...


Here it is the second meaning. The logic behind the sentence is:

众人回头观望，而这一看不要紧，所有人都是不由大吃一惊
The group looked back. This seemingly trivial action lead to a surprising finding.