It sounds like the Catharsis Hypothesis, which states that the release of aggression in small ways prevents large expressions of aggression. This has been rather thoroughly discredited. Small displays of aggression (e.g. ranting online) train an individual that aggression is an appropriate response, and leads to larger displays of aggression (e.g. physical attacks).

Source: http://www-personal.umich.edu/~bbushman/PSPB02.pdf http://blog.uwgb.edu/alltherage/four-questions-on-the-cathar...