not very familiar with these terms, but this seems to explain it:
Immediately after heelstrike, pronation can be further reduced by placing a firm "post" under the medial side of the foot. This (usually) foam block is typically 20 to 25 percent harder than the foam that composes the rest of the midsole. As the foot rotates inward, the change in midsole materials from soft to firm slows the rate of pronation. A medial post is most effective when located
directly under the ankle bone, where the impact loads and torques are transmitted from foot to leg.

http://www.runningtimes.com/issues/04nov/shoetech.htm

While some trail shoes are better equipped to help trail runners with motion control issues, the variable terrain makes each step different, so the standard pronation motion isn't as significant. In fact, shoes with strong medial posting can lead to foot misplacement by depriving trail runners of the agility required for maneuvering along twisty singletrack, especially on rock and root-riddled



http://www.runningtimes.com/shoes/04web/shoefaq.htm

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Note added at 12 mins (2005-09-14 20:39:15 GMT)
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