sailquik said..OK. Milsy is partly correct. I classify slalom gybes in two ways. 'Strap to strap' and 'Step'. Mine is a modified S2S. There is not another generic description of it. It is in the S2S category, as it NOT a 'Step' Gybe.
Truth be told, my step gybe is not as good
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so I seldom use it unless using a big sail, overpowered in rough water or when marginally planing (can't plane through). I find that at high speed on flat water, the step gybe has much more tendency to upset the glide and balance of the board as it almost forces you to get your weight very far forward on the board as you step, and then you must drop the rig towards the tail a lot. This results in a rocking of weight from back, to forward, to back again, slowing the glide. In an overpowered situation in rough water, this is not as important as pure control, which is why it is mostly favored by slalom racers.
In the 'pure' strap to strap gybe that Milsy describes, the old front foot goes to the centre of the board between the straps. Your full weight goes onto it as your old back foot is inserted into the new front strap as you sheet in. Then all the load from the sail goes onto your front foot as you put the back foot in. This works well on a wave board or another draggy board with the center of resistance well forward, but on a slippery slalom board the center of resistance is well back towards the fin of you are still planing free. This means that if you try to put the front foot in and transfer all your weight to itas you sheet in, the board will bog down and you won't be able to rake the rig back and keep pressure on the fin to accelerate. So instead, I put the new front foot in the centrer between the straps to drive the board and get the back foot in to maintain that as the board accelerates. Then you can maintain drive as you slip in the front foot.
On a smallish slalom board in flat water at high speed such as in a Alpha situation, most of the best Alphas I have seen are modified 'strap to strap'. This also becasue in a high G turn, you don't have time to do the two stage step then flip, and if you do, you are more likely to upset the board and have a longer pause between power off and power on.
On my 40cm speed board I actually skip the foot on the rail at the start. I start the carve with my foot almost in the opposite front strap and parallel to the rail.
As I flip the rig I simply twist my foot a little so it ends up in the other front strap and sail away with both feet more or less in the front straps until I have enough speed to move my new back foot to the center of the board or the back strap. There is very little room for your feet on these boards and they bog down very quickly if you have any weight back so this keeps the weight well forward as the rig flips.
On my wave board I usually do the classic 'strap to strap' that Milsy described.
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In any case, you don't want to grab the mast if you possibly avoid it!