What is that "smell" in one's sinuses after blowing your nose, and what causes it?

Common when you have a cold/stuffy-nose, sometimes a sinus will briefly clear up, and there is a shortlived ammonium-like smell left over.

I say ammonium because it reminds me of those Dutch Ammonium-Salt Liquorice candies.

Not pungent or typically smelly, but sort of like if your nose/sinus suddenly blue-screened/404'd. What is that, and what causes it?