The shifting IS centrifugal. There is a clutch inside second gear that has a spring loaded arm. When the spinning forces override the spring, a little arm comes out and engages second gear. Since first gear is on a OWB and the shaft is spinning faster than the first gear, it free floats.
There is a member here, b0gh0s, that has a little locking hub part that will lock second gear, but I don't think he visits very often. I used this method instead and has been working rather well:
I file little half-moons into the metal part and then screw some 4mm cap screws into the plastic gear.
Actually, you can see the little arm and catch pin in the pic even though they aren't used.