I'm not a biker but as far as I remember, it's a big bolt/nut that stops the rider pulling the accelerator the full way. Replace the big bolt with a standard bolt/nut and you're good to go. When it was explained to me, I remember thinking only the Irish Govt could come up with something so stupid.
Maybe one of the ex-couriers (or someone going through mid-life crisis who bought a bike) could explain better?