u c, for the forward gear, the gears for ratio 1-5 is in motion all the time.. all of it.. what determine which gear u are engaged in is the syncromesh..when the syncromesh engage, only then the selected gear engage.. for the reverse gear, it requires one extra gear to make the reverse direction.. since it is sharing the same output shaft.. the gears cannot be in motion coz it will deter the output shaft movement.. so a pinion gear is slide in between the driving and output gear to engage into reverse gear.. thats why it has to be spur gear (straight cut).. bcoz if helical than cannot slide in liau.. hope this explains it..
sorry my skill using paint is not that good..