This is a screen capture at Cal Maritime of a Transas simulation of a Damen 2411 Z-Drive tug escorting a tanker. The tug controls are in the lower right and the red oval indicates the towline force generated by the tug. Ship's speed is about 6 knots.
The pilot orders the tug to "Power Indirect My Port, Jacknife When Necessary" because he/she wants to assist the vessel in turning to starboard or the ship's rudder might have failed to port. Deck edge immersion is not recommended but may be acceptable in some emergency cases.
The red oval indicates the towline force, the green oval is the tugs speed and course over ground. "Power Indirect" indicates that the towline should be as close to 90 degrees as possible without damaging the tug, ship or line. During the jacknife maneuver the goal is to keep a strain on the line. This requires a fast winch to heave the line in as the tug turns.
The tug controls indicate direction and power order to the z-drive units.