Airsnort and Breadth Setting

I am a little confused as too how the breadth works ( and the Fluhrer
et. al. paper for that matter)? From what I have read on message
boards and looking in the code (crack.c). The higher the breadth the
more times it the checks IV? Why do you check the same IV over again?
If it doesn't reveal anything the first time why try it again?

I have cracked both 64 and 128bit keys with the default breadth
settings. Now if I set the breadth to 4 and 3, it seems to be taking
longer to crack a 64bit key. If I want to set the breadth higher, am I
supposed to update both values. Looking at the CPU usage nothing
really changed when I just updated the 64bit value. Only when I
updated both values then did I see the CPU usage escalate.

Does anyone know of a paper that eplains the Fluhrer, Mantin, and
Shamir attack in an easier format?
Thanks
Mike