From what I understand it's when you act without being able to control your urges. You don't do it because you are necessarily horny but because something in your life trigger a need, a compulse to masturbate. For instance with me it's being sad and depressed.
I understand it this way as well, when you lose control. You are not in concious mode, but react almost instinctibly, no logic, no thought, it's a compulsion.
A compulsion is a repetitive behaviour or mental act that you feel you need to do to temporarily relieve the unpleasant feelings brought on by the obsessive thought. it basically means you have obcessive thinking patterns and you feel you need to do something to relieve you from the unpleasant feelings . in this case porn
