Whats the idea of cutouts in slalom boards

reduces tail area that contacts the water surface when planing. Though at speed you want to be riding mostly on the fin the bottom tail surface of the board would be touching the water with trim changes and oncoming chop. All the wiggly woggly spaceship bits above that I don't believe do anything except look fancy (as they don't go near the water) for the new year models and possibly cause cavitation/trickier gybes :P Smaller boards are generally more and more on the fin and have less tail area anyway so I think differences would be less noticeable?

Bit of a cross thread response to your query on the Fanatic Falcon Sue, but having ridden iSonics for about 5 years I got myself a 2013 89 Falcon about two years ago and found that without the cut-outs the board was incredibly flighty going for it in the ocean, so much so that whilst an extremely fast board it took every skerrick of effort to keep it altogether on the water compared to a similar sized iSonic. I sold it after trying numerous different fins and set-ups as it just wanted to buck me off going broad (maybe user error too). Ended up replacing it with a 2014 iS90 and haven't looked back. Whilst the latest model smaller iSonics don't technically have cut-outs anymore their radical pin tail is effectively doing the same that a cut out previously did but only better IMO.

So all in all I really believe the tail cut-outs are a big plus to a boards ability to trim itself and thereby making the board more controllable. Also less wetted surface area reduces drag although compared to the drag a fin generates it's probably quite negligible whether there's cut-outs or not.

Here's the 2014 iS90 tail next to a 2012 iS97 FYI

They are there to reduce wetted surface area and therefore drag. A planing surface produces the highest amount of lift from its leading edge and the least from its trailing edge. So in theory you can reduce wetted surface area which is drag from the rear of the board without having a big impact on the amount of lift the surface generates. It does impact trim though and that is where it gets tricky,the cutout size and shape needs to be designed in as part of the overall balanced concept.

I don't claim to be any sort of authority, and I basically agree with most of the above, but here's my thoughts on it.

Providing a constant and optimum planning angle, the wetted area will automatically reduce with speed increase, cut outs won't affect this, it just puts the centre of lift further forward.

So in my opinion cutouts are there for trim.

Ideally the centre of lift should be under the load, then optimum angle of attack by adjusting mast foot pressure or front/back foot weight.
But as the centre of lift moves back with speed increase, (front of board lifts first), the centre of lift can move behind the load if the tail is too wide.
This makes it very hard to maintain the ideal angle of attack, lessening it and increasing wetted area by pushing the nose down.

The point this happens depends a bit on rider weight and position, it's going to happen earlier for a lighter person and a more forward footstrap position.

now there are not only cut-outs, but also flares/overhangs ?? like this RRD X-FIRE