When Henrik Mortensen and I cast 2 years ago I pointed out the names of all of these casts he was doing and he smiled knowingly at me and said "well, they are all just underhand casts with a change of direction thrown in" and of course he was right. I think we could create a lot of new casts by splitting hairs and saying this or that is different than the other so it is an entirely new cast, and on one hand I think that is fine for clarity but on the other hand I think it is bad for clarity because it can confuse new casters (or not so new casters like me!
As I understand it the switch is simply forming a live line D loop and casting the fly back to where it started, the basic improved roll cast that is the building block of spey casting. Pivot about the hips a little to make this same cast at 45 degrees to the flow and now you have a single spey. There are a bunch of different ways to make the single spey, and some have distinctive characteristics (like the Grant Switch for example), but I think it can get down to the splitting of hairs. I have heard the name Devon Switch before but I have never seen a description of it and haven't had it demonstrated to me so I don't know what it is exactly, but I'm guessing it is one of the single spey variations.
Case in point: I cast single spey with a combination of the Grant Switch, the underhand cast and elements of overhead casting, with a custom made extended belly spey line. It is distinctive and very powerful and I haven't seen a lot of other people do this (in fact, I haven't seen anyone else do this), so I guess I could lay claim to a new cast--can I call this the Thompson Switch or the Thompson single? Perhaps, but I just call it a single spey. Now, I might be wrong in approaching this as a generalist, but my point is not to minimize variation, rather to minimize distinction for the sake of anything other than clarity. There are a lot of people out there who would like to have us believe that a small difference is really a very big one, who would also have us believe that their way is the right and indeed the only way to do things, and of course I have concerns about this approach when it comes to spey casting, something I've made no secret of these past 3 years (of course that doesn't mean that I am right, either! .
) Maybe I should be getting a little more specific about some of these things, like the Thompson Switch.
Startin' to kinda like that name now that I think about it.
When it comes to the FFF Spey Certification I think it is unlikely that they will throw an oddball cast like the Thompson Switch at you and say perform it; I think it is entirely likely however that if you are asked about variations of the single spey and you mention figure-of-eight single spey, Grant Switch and Thompson Switch you better be prepared for an examiner to say "can you show me the Thompson Switch and explain with demonstration how it differs from the other two you mentioned?"
Of course, maybe with the Thompson Switch you could just fake it and the examiner wouldn't know unless it was me!