That's not necessarily true; there are three types of popular pressure cookers[1]. Fagor-type pressure cookers release steam constantly (this could be the kind GP has), old-school weight-on-top pressure cookers release steam intermittently (this may be what you have in mind), and Kuhn Rikon pressure cookers release no steam at all (this third type is best for flavor since no volatiles are lost to the kitchen with escaping steam).

1. http://www.cookingissues.com/index.html%3Fp=2561.html