There are two types of reversing engines predominantly used on marine engines. The direct type has a single long stroke cylinder attached to a crank arm on the reversing shaft via a connecting rod. One end of this cylinder’s stroke puts the Stephenson gear of the main engine into forward motion, the other end of this cylinder’s stroke puts the main engine into reverse. The speed at which the reversing engine’s piston could move was usually restricted by adding a hydraulic cylinder similar to a shock absorber to the reversing engine to resist sudden moves by the reversing engine. Without doing research, I believe that this is the system used by Titanic. There is also a worm gear type where a small donkey engine with a worm on its crankshaft engages a worm gear equipped with a crankpin, attached to the connecting rod to the reversing shaft. In this type the donkey engine is run to turn the worm gear one half a revolution, to shift the main engine from forward to reverse. The Liberty Ship John W. Brown uses this system. Both would take perhaps three to five seconds to shift the Stephenson links.
In the Titanic movie, as I recall, the Stephenson links slam over to astern motion very quickly (less than one second?), surprising the oiler standing on the grating nearby. That struck me as odd at the time, in that the potential for damage of the valve gear itself would be likely in normal maneuvering if that parts moved that fast.
With regard to having the throttle open or closed, if reversing is attempted with the throttle open, the added resistance caused by the steam pressure on the valves can be enough to overwhelm the reversing engine, causing it to stall at some point between full forward and full reverse, leaving the main engines stopped until that pressure is reduced. Even though they are known as “balanced” and are theoretically unaffected by steam pressure, I can assure you that piston valves do have more resistance when under pressure, the same as slide valves, though to a lesser extant.
With regard to the main engine’s power train, from the pistons to the propeller blades, in a well designed plant, there really is little added strain in performing a full power reversal. The steam in the cylinders is elastic, and has give. The cylinders are equipped with pressure relief valves that prevent dangerous over pressure from developing. All of the power train parts are designed to safely carry the loads that those maximum cylinder pressures impose. The propeller itself is incapable of transmitting damaging torque back to the engine it that once a certain PSI of blade loading is exceeded it will simply slip in the water. If the propeller begins to actively cavitate, then potentially damaging vibration will develop, the engines will race, and the wise engineer will reduce throttle until the cavitation stops.
If the throttle is closed with the ship traveling at high speed, the propeller will “windmill” in the water and keep the engine turning over forward until the throttle is reopened to “back her down”. In my opinion, this was shown accurately in the movie. The one detail I loved in that sequence was at the point that the throttle was reopened and the engine came to a stop, all was quiet except for a slight creaking noise as the engine loaded up with torque in the opposite direction. That was a wonderful detail. My thanks to the sound department.