This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
Shields must allow some energy through to allow sensors to operate. To
be safe, these frequencies are cycled, allowing sensor windows. By
knowing the shield cycles, and the right frequencies, it is be
possible to adjust the transporter to work at those few open
frequencies, and slip past the shields.
Of course, if the destination ship detects you trying to beam through,
they can alter the shield frequencies and end the transport suddenly,
with rather messy results.
 
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