What does this t-stat bypass do?

I changed the thermostat last night on my jeep and the old one had this little bypass thing but the new one I got from Part Source didn't have it. What's it do and do I need it?

(this isn't my t-stat, but it shows the bypass i'm talking about)

 

Fail-safe perhaps? So that if it fails, it fails in the open position?

 

Nope, fail safe is a 1 way clamp thing that it opens into and clamps it on the sides. Not exactly sure what this little bypass does.

 

It's called a jiggle valve.

http://answers.yahoo.com/question/index?qid=20070602061734AAmPAqu

 

Thanks, +repped.

 

tx!