When can I activate the differential lock so that this does not possibly damage the motor vehicle?

You answered correctly. The differential lock should only be engaged when the vehicle is stationary or moving very slowly, such as at walking speed. This is because activating the differential lock at higher speeds or while turning (like when understeering in a bend) can cause mechanical damage to the drivetrain. The lock is designed for situations with low traction, such as off-road or slippery surfaces, and works best when the wheels are moving slowly and straight. Engaging it under the correct conditions prevents potential harm to your vehicle.