PvP Terrain Exploits

How do terrain exploits differ from regular PvP moves?  Cynwise talks about Blizzard's rule against exploits, how this applies to PvP and the problems inherent in figuring out just what is bannable and what isn't in the PvP world.
http://cynwise.wordpress.com/2010/09/02/the-walls-of-wintergrasp-and-terrain-exploits/

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