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.