I have read recipies that suggest letting the pressure naturally release when cooking grains in a pressure cooker. I've always just released the pressure quickly through the pressure relief valve.
What does releasing the pressure quickly do to grains as apposed to letting the pressure dissipate naturally through cooling?
The main reason for using the quick release is to prevent overcooking, think about what would happen to white rice if you left the cooker to de-pressurise naturally: it would be mush. Of course, you could factor the time taken to come back to normal pressure into the original cooking time but that's fraught with difficulty because it's dependent on what temperature it is in your kitchen and probably a multitude of other factors we're not aware of.
A quick pressure release will cause the pressurized water inside to boil.
By maintaining higher-than-the-outside pressure Pressure-cookers raise the boiling point of water - meaning you can cook your soup in liquid water at 220f. If you quickly release that pressure, that superheated liquid water will be able to turn to steam - AKA boil - and it will do so rapidly.
However, if there's not a major heat-source running while you do this, the boiling will probably stop fairly quickly. (Boiling is actually a cooling process, much like the evaporation of sweat.)
Depending on how hot it was, it could do some interesting structural damage to the food - if it was a stock, suddenly boiling it might make it cloudy.
That all said, odds are pretty good you're going to be OK with a rapid release.
