In *Prisoner of Azkaban*, when Snape examines the Marauder’s Map he says

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> “Professor Severus Snape, master of this school, commands you to yield the information you conceal!”
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> – *Prisoner of Azkaban*, chapter 14, *Snape's Grudge*
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> 
> 

This doesn't sound like much of a spell, and it seems like an odd thing to say when he’s not the headmaster.

Can anyone explain?