[BrianG]

Thank you suicideneil, I couldn't find a pic of the locking part and it is not easy to explain. This little part helps second gear last longer since there isn't that sudden snap from the play in the dog gear.

If you lock second, you can reduce weight by going with only one idler, and then shaving all the material off first gear so only a sleeve is left as a spacer to keep proper bearing placement.