Maybe "rail-centric" vs "tail-centric" is a better summation ??
It's very much the case that to get the most from the newer boards, the rider must employ a style/technique that focuses (more) on use of the tail block/fin combo and less on the overall mid section(rail line) of the board. Again, the differences are subtle - but real - and can significantly influece factors such as early plaing.
There are very strong efficiency advantages (better lift to drag ratio) in using a wider (and shorter) planing flat such as evolved thru FW and now into slalom. The secret is to maintain (thru design, tuning, rider skill and 1000 other variables) the longitudinal trim, because if you loose (control of) that, you quickly have a wide LONG planing surface which (you guessed it ) doesn't perform as efficiently as a narrow long one. With better (more draft stable) rigs and improved fins, there is (more) room to explore (and exploit) the potential of the (theoretically) more efficient L/D of the wider tail - and produce an overall "better" package (obviously, subject to personal vs. average or competition opinion).
With these wider tails (and "softer rails"), the other critical but again subtle design change is the rockerline, which needs to be adjusted such that the board (by nature of rocker) trims back at working/planing speed onto a shorter (but wider) rear planing flat than the same "size" (volume) "narrow" board. (OK, loads of generalisation here, but minimising the variables for ease of discussion). Basic fundamental objective of the Dual Flat Concept DFC is to provide a distinct break point in the rockerline to help "clear" the mid/front section from the water (at speed).
So once you have done that successfully (and got other criticals like jibe, control, bounce) under control, the mid and forward sections of the board (and rail) are a lot less involved in the primary role of "planing surface" (and working efficiency) than they were in a long, narrow board - and along the way have been "optimised" towards increased control (softer or more tuck) - which is not just a safety or comfort factor, but allows the rider to push on when previously (or otherwise) they would start to reach control limits in both a mental (rider) and physical (board) sense.
Important differentiator too is the subtle difference between description of rail and tuck. And how (or where) it varies along the rail length, in relation to the boards outline (short+wide vs long+narrow). In general terms the "rail" defines the whole cross section of the edge of the board, the radius or outline of the cross section from deck to underside. In more specific terms, it can define the actual join (interface) of the curved side of the board to the flat underside, the actual "edge" or corner. In all slalom boards the rail to bottom/underside intersection (at least at the tail and mid "working section" will tend to have a clearly defined "hard" (corner) release edge to ensure a very clean (efficient) release, (compared to say, a fully rounded and soft "edgeless" Malibu board rail). However, by increasing the tuck on the rail before it reaches the underside, you effectively reduce the "bite" of the rail (especially when considered in a lateral or sideways movement).
( So what ?? Boards move forward, not sideways… Exactly ! But when you pump (even modestly), although the result is to push the board in a forward direction, the pumping action of the rider against the board/rail/fin is trying to move it sideways. And when (moments before a deadly rail bite at speed) the board slips or slides, that rail if effectively moving….sideways!
Of course these are the two extremes (sub planing and spinout ) - in between the differences are there as well. )
iSonics are (by comparison to say even Futura or S-Type) still relatively hard railed, so this whole discussion is really on very fine subtleties.
Some more quick comments on some of the other discussion points above..
The iSonic range during evolution and development benefits from a lot of comparative testing against older model designs, often in marginal slalom conditions (Thailand), and by light riders such as Tiesda and Per (RIP), not just the heavyweights (in a physical or political sense). Yes, it's true the Antoine's and KPs have a serious input and yes it's true these are racing machines in constant evolution to remain at the top of their game, but at the same time the "roots" of this development have serious light weight test rider involvement.
It's also realistic to note that a 70Lt board optmised for use in 40+kt conditions by a 100Kg A2 is probably going to be (ideally) a little different from a 70Lt slalom designed for use by a 50 Kg Japanese lady racing Kanaha in 20 kts. Both would be on the right "size" board. A key factor here would be water state / conditions variance, as well as the obvious rider weight/trim factors.
Increasing the range of the iSonics (both the boards basic wind range, and it's ability to perform better outside the "ideal" sail size range) is a huge practical benefit to the average sailor - and the sport - not just top PWA pros racing under "2 board" restriction. The general trend across the sport in recent years is towards more (but better or well behaved) volume (for a given set of conditions) to improve range/versatility etc. That benefit doesn't stop at iSonic, look at Futura and even Kombat and beyond.. One of the extinction factors from the previous slalom boards was very clearly their narrow and very "edgy" range ; fast as hell in a very narrow range, and generally requiring massive skill and cojones (and maybe luck) to get the best from them in this narrow range.
The slalom market has grown and matured in this current "cycle" and the average customer has become more advanced, more skilled and more demanding of range and top end performance. On average, they are willing (or even demanding) to exchange a need to refine/update/modify their style or technique to take advantage of higher performance /range potential.
I'm personally from the old school of super narrow and massively hard railed slalom/speed boards, and can relate explicitly to the "efficiency" that Geo and others discuss. These older boards - with the right skill, application and conditions - could do some amazing things and bend a few rules. Still do. But by comparison to the overall "package" the new boards offer, their versatility is significantly restricted.
(anyway, now it's windy and time to go sailing)
Cheers ~ Ian