Subject: why doesn't magnetic shielding work in this situation?

in the previous question from
http://www.madsci.org/posts/archives/aug98/897772938.Eg.r.html
i understand the concept here that the energy required to do these type of 
motors is more than it can produce, but i don't really understand magnetic 
shielding , and i know it wouldn't work, or someone would have done it 
already, but why doesn't it work to use magnetic shielding to shield the 
magnets field in places where it would push the wrong direction, and not 
use shielding where it would push it the right direction.  I'm not sure if 
I expressed myself very clearly, but anyway.

Re: why doesn't magnetic shielding work in this situation?