Kezza...I'm certanly no expert on all this..but the stepping back to turn is not a rail thing..(I don't think)..It's more to lift the nose a little using the tail rocker and reducing the area of the board touching the water..the shorter the board or contact area the quicker the board will turn..I often wonder about the rail thing..My guess is that using the oppisite rail at that slow speed would be best for a quicker turn..Sorta like the old windufer flare jibe.
I think if you can paddle with enough speed you might be able to carve a turn using the inside rail..But I tend to drag the paddle on the inside of the turn and slow right down before putting the paddle on the other side and do a wide sweeping paddle strokes..this gets the board around quicker..for me.
So yes..sinking the tail is providing a pivot point.
Also re going straight I would use about 3-5 strokes each side before swapping..although I can paddle continuisly on one side and hold a straigh line..But you soon get tired on that side and need to change sides.
Some people find they are stronger on one side..maybe right..and so they might paddle 6 times on that side and only 3-4 times on their left side..That sorta thing.
Also re the turning thing..Check out this vid..at about the 1.22 mark Ivan does a great sink tail pivot turn..Does it look like he uses one rail or another..I don't think so.
I hope that helps some..

..others might like to add their thoughts on this...
Here's the vid...
://www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=37290
DJ