A power-driven vessel "not under command" must show her sidelights when:

A power-driven vessel that is "not under command" is defined as a vessel that cannot maneuver as required by the navigation rules, typically due to issues such as a mechanical failure. According to the Rules of the Road, such a vessel must exhibit specific lights to ensure proper visibility and communication with other vessels.

When a power-driven vessel "not under command" is making headway, which indicates that the vessel is moving through the water, it is required to display sidelights. This is crucial for safety and navigation, as sidelights help indicate the direction the vessel is facing and ensure that it is properly visible to other vessels in the area.

Displaying sidelights while making headway is important to prevent potential collisions, as it allows other vessels to ascertain the status and orientation of the "not under command" vessel. Therefore, the correct and safest practice is to show sidelights when the vessel is in motion, thereby contributing to overall safety on the water.