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

The differential lock should only be activated when the vehicle is stationary or moving at walking speed to avoid damage. Engaging it at higher speeds or while the vehicle is turning (such as understeering in a bend) can cause excessive stress on the drivetrain and tires, leading to mechanical damage. The rule is to use the differential lock only in low-speed conditions where wheel slip is likely, ensuring safe and damage-free operation.