Honda TRX420FA 2009 Owner's Manual page 80

Shifting Gears
The transmission cannot be upshifted from neutral to 1st gear when the
engine speed is above 2,300 rpm or the ground speed is above 2 mph
(3 km/h).
The transmission cannot be downshifted from 1st gear to neutral when
the ground speed is above 1 mph (1.5 km/h).
If the ground speed in 4th or 5th gear is decreased under 6 mph (10
km/h), the transmission is downshifted from the current gear to 3rd gear.
If the ATV stop completely, the transmission cannot be selected over
4th gear or 5th gear.
If the electric shift system malfunctions, the transmission cannot be
shifted by pressing the gearshift switches. See your Honda dealer. (In an
emergency, a gear may be selected manually so you may move the
vehicle. See Emergency Gear Selection & Operation,
Learning when to shift gears comes with experience. Keep the following
tips in mind:
As a general rule, shift while moving in a straight line.
Close the throttle completely before shifting. Improper shifting may
damage the engine, transmission, and drivetrain.
Upshift to a higher gear or reduce throttle before engine rpm (speed)
gets too high. Learn the relationship between engine sound and the
normal shifting points.
Basic Operation & Riding
72
page 192 ).