Mike,

As I commented earlier, we were sailing at Datchet in a VERY large (and deep) concrete bowl. In earlier races I sheeted the sail quite tight downwind as I would normally do, but found that as the wind swirled and sheered the boat seemed to 'die' at times. Easing the mainsheet slightly seemed to get the boat going again as did - rather strangely - sheeting the spinnaker in tighter. The boat just seemed happier with more mainsail twist than I would normally sail with on open water so that's what I did for the rest of the weekend

Like I said before, the vid is far from textbook

Wouter is right though, there is a definite rhythm to downwind sailing and it is much easier on open water where the rhythm of direction changes is more predictable. You need to overlay the wind direction changes with what the waves are doing so that downwind is a combination of surfing and optimum sail angle. Rudder movements need to be small and timely but as the wind/waves get up you can be more aggressive with the helm.

It's practice followed by practice and, oh yeah, practice!