It's tied directly to levels of arousal and the stimulation I receive.
High arousal or High stimulation makes me shoot. If both are high then the quantity significantly increase and the distance shot is legendary. A case where quantity and the shooting is tied directly to the girl and her performance... or mine if I am fapping. 2 hand stroke where one hand is going down the shaft followed by the other in an infinite loop is a sure fire way to get me to blow, jerking is definitely not a guarantee.
If both are low.. it's sad. Oozes out and not much if any comes out.