Method of allylic and benzylic oxidation
This is one of those heterogenous reactions occurring on a surface which are  mechanistically messy. As expected for a reaction that occurs on the surface of a reagent, factors such as surface area and reagent preparation (freshness, surface contamination) are significant variables.  A free radical mechanism is generally accepted and a reasonable mechanism is posted below.  



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When an allylic alcohol with stereochemistry (cis, trans) around the double bond is oxidized, the stereochemistry around the double is preserved.  



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This suggests that we are not really dealing with a free, long-lived radical intermediate, but rather a free radical held in close association with the manganese ion.