RE: rig action on a bottom turn
Beinn the sucker for analysing everything from boards to riding, I've looked quite a bit on both Kevin and others. One thing to understand is that most people adjust their style quite a lot relative the conditions. That said, there are often som elements of style which you can identify regardless of consitions.
As for Kevin, I would no say he typically rakes his rig backwards that much, but you're right in that he tend to oversheet quite a bit and also maybe rake bak a little just when initiating hte turn. Many people do this a lot but others do it less. If you look at Scotty, he generally enters his bototm turns less oversheeted and with more sail power on.
Style is very individual and all these top guys are natural talents and I doub't every separate move is done for a specific reason. Still, I would say that the reason for oversheeting a bit in the start of the turn, is to kill power as Kevin says, but also to get in a neutral and natural position over the board at this stage. A "modern" so called front foot bottom turn is largely characterised by that the rider is sort of gradually rotatng the sail in the opposite direction relative the turn itself and laying down the sail forwards. To do this it makes sense not to start out in a sheeted out position.
Another way of explaning this is that during the drop in and acceleration phase on a fast wave, you want to stay as neutral as possible on the board and also keep the sail neutral. This is especially evident in super light winds, when it can even be hard to accelerate down the wave at all without carefully "tucking in" the sail. But when you're at the bottom of the wave, speed is so much higher and the apparent wind makes using the sail possible again.
If you're aiming for a so called from foot bottom turn, I'm a bit hesitant to use the explanation by Russel. Its always hard to put these things in words, but rotating the rig backwards, towards the tail, to get pressure on the back foot leave you in a kind of difficult position if you later want to engage more front rail. But again, it depends on the wave too. On the nice video Russle posted, I would say the techique mostly used is "half back footed". You can see that the rider is sometimes leaning back over the tail instead of forwards to engage more rail. But it seems to fit the waves at that spot.