The documentation of movePrimary contains several cryptic warnings when it should or shouldn't be used, but doesn't actually explain what it does or doesn't do.

http://docs.mongodb.org/manual/reference/command/movePrimary/

What it actually does is move all unsharded collections from the old primary shard to the new primary shard, but it does so without any synchronization or other protection, and therefore the command cannot be used on a live system, since the results of reads and writes on the affected database would be undefined while the move is in progress. This needs to be explained clearly in the documentation, while some of the warnings (such as, "only use this when removing a shard") should be dropped.