You are right but in practice gear does dicide the available power at that speed. Gear selects the amount of revs and revs decide the amount of available power!
Lets say at a certain speed third gear gives you 2000rpm with 80ps and second gives 3500rpm with 150ps. It would be better to be in second approaching a trafficlight.
For safety you should allways be in the gear with maximum power.
Especially going through a bend in neutral is just the stupidest thing ever. You're never going to pass your exam like that. (never give advise if your not sure)
Maybe for Americans a manual is some increadibly complicated thing but it's nothing special.