The ability to shift from two-wheel drive to four-wheel drive while a vehicle is moving is called shift-on-the-fly. The phrase describes a drivetrain designed to engage four-wheel drive without requiring the driver to stop and leave the vehicle.
In a conventional part-time four-wheel-drive system, the transfer case sends engine power to both the front and rear axles when 4WD is selected. A shift-on-the-fly mechanism uses synchronized gears, an electronically controlled actuator, or another compatible transfer-case design to make that transition while the vehicle is in motion. It is especially useful when road surfaces change quickly from pavement to gravel, mud, snow, or sand.
The term does not mean every four-wheel-drive setting can be selected at any speed. Many vehicles allow shifting between 2WD and high-range 4WD while moving, but require the vehicle to stop—or move very slowly—for low-range 4WD. Shift-on-the-fly is also different from all-wheel drive, which usually manages torque distribution automatically rather than through a driver-selected transfer-case mode.