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[fazersharp]

There is a neutral position for the selector drum between every adjacent gear (obviously one gear has to disengage before the next engages). They can make it available by putting a detent at a halfway position, thus a full movement of the gear lever will go past the neutral and a gentle nudge will let it settle at the mid position.
You could have a drum designed for a halfway detent below first, but the danger is that you'd change down for a tight bend having misjudged which gear you're in and get neutral rather than first.

I knew some one would give a explanation that I didn't quite understand- thanks anyway  :lol