Tunneling? Or maybe a combination of this and a thing from fluid dynamics called the venturi effect. If two cars enter a tunnel at the same time, it's okay as long as the width of the tunnel is sufficient to accommodate both cars. But if the tunnel gets narrower and narrower they will smash into each other. Using the venturi effect, the cars get faster and faster as they go through the narrowing opening. When they smash together, instead of being destroyed, they merge into one car, or in this case, one atom of helium, and the "leftovers" become energy released as heat. Now that they are "one" car, or "one" atom, they take up less space than two cars or two atoms, so there is now room for the cars/2H atoms behind them to do the same thing in sequence, until you run out of cars/2H atoms.............But that's just my guess............
Well, the Venturi effect exists due to the disturbance of a fluid medium between the cars (the air). At nuclei scales, there is no medium for the Venturi effect to occur in.