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It doesn't actually "work" on 2.4.5 – it runs the explain on the wrong query. See Scott's first comment in SERVER-6767 for an explanation.
To see in action:
> db.foo.find({_id:0}).explain()
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{
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"cursor" : "BtreeCursor _id_",
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"isMultiKey" : false,
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"n" : 1,
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"nscannedObjects" : 1,
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"nscanned" : 1,
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"nscannedObjectsAllPlans" : 1,
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"nscannedAllPlans" : 1,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nYields" : 0,
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"nChunkSkips" : 0,
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"millis" : 0,
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"indexBounds" : {
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"start" : {
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"_id" : 0
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},
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"end" : {
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"_id" : 0
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}
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},
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"server" : "Rassi-MacBook-Pro.local:1338"
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}
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> db.foo.find({$query:{_id:0}}).explain()
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{
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"cursor" : "BasicCursor",
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"isMultiKey" : false,
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"n" : 0,
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"nscannedObjects" : 1,
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"nscanned" : 1,
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"nscannedObjectsAllPlans" : 1,
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"nscannedAllPlans" : 1,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nYields" : 0,
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"nChunkSkips" : 0,
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"millis" : 0,
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"indexBounds" : {
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},
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"server" : "Rassi-MacBook-Pro.local:1338"
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}
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