What is a Sector Plate? The following is from "Layouts Using Sector Plates" -
"A popular space-saving device is the sector plate -- a track or tracks mounted on a board that is pivoted at one end so it can match up with several approaching tracks."
A Sector Plate is in the same 'family' as a turntable. A turntable is a length of track pivoted in the middle. A Sector Plate is pivoted at one end - so think of it as a turntable with the bit beyond the pivot whacked off on one side.
There are several uses for using a Sector Plate on your layout. A layout of any size can use them for a fiddle yard where the Sector Plate replaces a yard ladder. For micro-layouts they provide a way for you to have operation in a very small area. Sometimes they are hidden in a building. You see two tracks entering the structure but what you don't see is the Sector Plate hidden within. This provides the ability to switch a loco from one track to another and all you need is a length of track long enough for that loco.
Here's some links to look at:
Layouts Using Sector Plates
Layouts Using Sector Plates Pt 2
Indexing sector plates - the second article down titled "and still more fiddle yard tips!" talks about two ways to index sector plates
Turntable & Sector Plate Fittings - This is a commercial site selling fittings but worth looking at for ideas
Video of a Z scale sector plate - this YouTube video shows a Sector Plate in operation. The relationship to a turntable is obvious. It might be argued that this example IS a turntable I suppose .. not that it matters in the least.
I know that Tom shows a Sector Plate on his plan and I would hazard a guess that Ian has some knowledge of these. Since my On30 layout is seriously lacking in operation with the current design I am planning to add Sector Plates in the future. What I am going to look at is a fiddle yard that folds up out of the way when not in use at the end of the "L-shape" foot .. and possibly another hidden in a building.