The Ejaculation starts when Widowmaker is at that the Top of the Onahole, So due to the Pressure by the Forward Thrust, the Empty Space by the Shaft, where the cum temporarily flows to(the dick isnt a perfect cylinder), and the glans creating a plug, the cum has a pressured window to exit from, ik it looks wierd but its "realistic", its like a leaky piston in an engine due to a gap in the walls of the engine(causing fuel/gases to leak at the bottom, here its upside down), my bad for the explanation, i am too lost in da sauce xD