The term was coined from the conventions of Greek tragedy, where a machine is used to bring actors playing gods onto the stage. The machine could be either a crane (mechane) used to lower actors from above or a riser that brought actors up through a trapdoor. Preparation to pick up the actors was done behind the Skene (theatre). The idea was introduced by Aeschylus and was used often to resolve the conflict and conclude the drama. Although the device is associated most with Greek tragedy, it also appeared in comedies.

It is generally deemed undesirable in writing and often implies a lack of creativity on the part of the author. The reasons for this are that it does not pay due regard to the story's internal logic (although it is sometimes deliberately used to do this) and is often so unlikely that it challenges suspension of disbelief, allowing the author to conclude the story with an unlikely, though perhaps more palatable, ending.

1 and 2: See the recent discussion of Weathertop here. There is no really satisfactory answer to why the Riders didn't complete their task there.



a) The WK was dismayed by the resistance that Frodo showed and in particular the uttering of the name Elbereth
b) He recognised Aragorn as a "power" but did not know his identity.
c) He thought the Morgul Blade would do it's job of turning Frodo into a wraith much quicker than it did, as they knew nothing of the nature of Hobbits, so his hope was of a further ambush with a turned and therefore submissive Frodo in the near future.
From a pragmatic and dramatic point of view with the ever wonderful benefit of hindsight these probably don't seem like valid reasons, but this is one of the beauties of Tolkiens writing. With great subtlety he introduces the fears and failings of the very powerful. The WK thinks he has done enough, so he departs for safety. Even he is not immune to fear and doubt and is only driven on by the terrible threat of his masters will.