G'day Dale, I dont think 'Strength' is always a major factor. Sure, I was strong in my younger days, but I sure aint now that I am 68.

I always set up my gear to take all the strain off my body as much as possible, otherwise I wouldn't still be able to do it.

That said, before going to Luderitz in 2013, I spent the whole year going to the Gym to try to regain some core strength and upper body strength, as I anticipated that I would be spending a lot of time hanging under the boom trying to waterstart with 8-10Kg of lead on. It was true, and it paid off,

Fitness is important for sure, as the young fit guys at Sandy Point like Kato, Ado and Mat sail upwind a lot more than I now can on the big days and get more runs. They are also more likely to be in the right place when a squall comes through. Sandy Point in a storm is very physically tiring.

On the other hand, this year after a couple of weeks at Lake George, I was fit enough to be able to circulate speed laps for well over an hour, but the conditions there are very different.

IMHO, it's more about setup and 'feel', and then the subleties of leverage and stance come into play. It's the whole package. If you compromise on the sail (oh I cant be bothered going in and changing now the conditions have changed, i'll just crank on some more DH), or the fin, (same) or the board (same), you my never be on just the right combo at the right time. I know quite a few people who 'just go out on what I know I can get back on', or, 'the fin that I know will be easy if I have to waterstart in the thickest weed', or 'the fin I will be able to get back upwind easier', rather than what is ideal for speed on the actual run. Speed sailing is very tactical and punishes compromise, lack of commitment or lazy. The guys I mentioned above, will often make the extra effort rig two or more sails, and have a couple of different boards sitting on the beach just so they can take advantage when the conditions change, or if they find what they are on is not working ideally.
This is also where all the subtle, refined skills come together. You can ride the smaller board, or the more effecient fin with confidence because you know you can jibe it without falling in and having to waterstart in that thick weed patch, and sail upwind efficiently on that gear and not end up stranded on the downwind bank, etc, because you have had the drive and commitment to do it so many times, that you have those skills down now.

The last thing is harder to describe. It's about 'feeling' the power and pull of the sail and the best angle to sail in the gust. A big one, as Kato mentioned, is to try to minimise body and sail movement. Smoooooth as you can. Board as steady as you can. Dont overmuscle it, Don't oversheet it, relax your back leg slightly so you dont shockload the fin.
Feel the sweet spot. Flow with it, don't try to force it.

Sometimes, in PB conditions, it's normal to be ****ting yourself,

but focus, trust, relax and don't panic.