Thank you Gregg, french helps!

My impression is that it's a plot device that would be regarded as wholly unrealistic, but is used nonetheless to move the plot in the desired direction -- perhaps to get a character out of trouble, or to bring two charcters together, or to resolve a major conflict, etc...  Perhaps it helps to think of it as slang for Deus ex machina? In this usage, the invisible God uses a crowbar to pry open the plot and do what she or he wants.  Somewhat of a trendy term...