This is from the owner's manual for the L3301, L3901 and L4701, it doesn't say anything about engine speed:
Synchro-Shuttle Shift Lever
Shift the shuttle shift lever forward to obtain forward
speeds and shift back to obtain reverse speeds. When
changing the shuttle shift lever, depress the clutch pedal
and stop the tractor before shifting.
The synchro-shuttle shift lever may be shifted while
the tractor is moving slowly and the clutch is
depressed, but sudden gear shifting may cause
transmission damage.
When you stand up from the seat with the shuttle lever
at "FORWARD" or "BACKWARD", the engine will stop
regardless of whether the tractor is moving or not. This
is because the tractor is equipped with Operator
Presence Control system (OPC)