I just came across an interesting description of a writing style called "Plot Coupons":
Sounds just like the next Harry Potter book, eh MaxiNibs? Neil Gaiman also said the first Sandman arc and Neverwhere are "Plot coupons." Can you guys think of any other "Plot Coupons" books out there? Let me know, they are my favorite types of books!A plot coupon is an object whose possession or use is necessary in order to resolve the conflict upon which the plot hangs, when this necessity clearly springs from the arbitrary decision of the author to make it so necessary. (The name is derived from a joke: when the characters have collected enough plot coupons, they can trade them in for the denouement.) For example, if the main plot of a novel concerns an ancient artifact that was broken into several pieces, and which must now be collected, reassembled, and activated in order to defeat an alien threat, the separate pieces of the artifact are surely plot coupons.
There is also something called a Plot Voucher, read all about it here: