Trying to understand more about manual's, and came across this
excellent article:

http://auto.howstuffworks.com/transmission5.htm

However, here's the part I don't get: What is preventing the driver
from shifting into reverse while going 60 miles an hour?

>From the colored diagram.... Please correct me if I have this wrong:
The purple collars are always spinning when the car is moving. The
blue, red, and green STOP spinning when the clutch is in neutral and
the clutch is pushed in, or at least, spinning slowly from the friction
of the output shaft (yellow) rubbing on the bearings that the blue
gears ride on.

TIA!