I'm the guy who coined that term, but it was a parallel evolution and a lot of guys experimented individually in the same timeframe- which eventually resulted in some fantastic lines and greater lining options. The two best production Skandit lines that came out initially were the SGS Scandit and the Steelhead Tactical for those who liked a bit more length.
It arose once the limitations of the beercan Skagit became clearer to us with time and usage. Some of us were looking for a versatile shorthead that would not only carry moderate sinktips and bigger flies, but enable sweet dryline casting too with a floating tip. A crossover between SA and T&G, enabling comfortable use of both methods with one line.
I'd call it less of a distinct casting style and more a set of gear choices that ends up maximizing versatility for your basic outfit, and shaping your loop to suit after that. It's particularly useful when fishing moderate distances over rivers that ask for flexibility in presentation, such as moderate-sized coastal rivers in OR or the OP, where I used it most with good result. Shallow tailout? floating line and underhanding. Brushy banks with a deep slot? Sinktip, moderate junk, sustained anchor with slightly opened stroke. Beautifully versatile. Less crap to carry when you're on foot, too.
The basic head, later called a driver, needed to be heavy enough in a relative sense (or it wouldn't carry a sinktip easily), short enough but not too short (for nice anchor and flight characteristics in scandi mode), with a cut point resulting in adequate tip diameter to turn junk over when in SA mode, but still giving a fairly light and mobile feeling when fished with the floating tip.
I started fiddling with Skandit as a concept in '07 myself, and by that winter had arrived at a fairly light pseudoscandi taper with a cut point that allowed 120 grain sinktips, but still enabled beautiful scandi casts with the floating tip (albeit with slightly slower line speeds and less compressed stroke). I wasn't alone in this, of course, folks were hitting the problem from lots of angles.
I broke my needs for the rod into components then built up the lines to suit. After a few very frightful frankenlines, I started hitting my own personal sweet spot with the design. I made some experimental lines in multi-D versions too, and had a floating version in circulation as a loaner at one point (that one was a heavy sh Outbound head, cut off and flipped end for end). Not long after, manufacturers began making some beautiful line choices available so I began using them.
Most of the time when I see Merkins casting a scandi head, they're using a more open cast, less bottom hand with more A>B travel, and it's often much heavier than an Andersson style caster would use on the same rod. I see very little disciplined Underhanding. (Capital U because, in the words of Mr Bencivenga, "all spey casts are underhand, dammit"- and he's right). There are numerous kinds of "scandi" styles, most prevalent in use is the Modern style, which is a bit more open and less compressed than Andersson style, which most Americans think of when they hear the word Scandi. But make no mistake, all are deadly. btw, yes, longheads can be and are Underhanded too! but that lies in the gray zone that we always want to avoid for some reason, and is a topic for another thread. There is a point where Longbelly meets Scandi, and that's a fun place.
There are a lot of twists in Scandi world, and to Scandinavians fishing Scandi gear, all of this is old hat! I could have shortcut my own development process by simply getting a Swede or DD head, cutting it back to a slightly heavy feel for Underhanding, than whacked the tip off at .050 for tips. As they say, hindsight is always 20/20.
In any event, if you're a DH fisherman on a budget and just sort of finding your way right now, Skandit may be useful to you. Cheap, versatile, easy to use, forgiving of casting imperfections. There's higher refinement in these heads now, and the length/weight/taper ratios are very favorable to Skandit with many of the shorthead lines available now; OPST Commando, shorter Nextcasts, SGS Scandit, Airflo Skagit switch-- it seems like every manufacturer has a line that will work as a Skandit driver now, and that's a good thing. Line evolution is making shorthead style distinctions less relevant; it's just a matter of getting the head length/weight that suits your preference and will fish the way you want it to.
A good way to roughly estimate what you need is to split the difference between the Scandi head and the Skagit head preferences on a given rod. I'll just knock 50 gr off the Skagit weight and go with that; or more often,add 30 gr to my Scandi head preference. Even better, pick up an SGS scandit package and you're done!