Thanks, George - I tried that search but can't find the clip. Doesn't matter, though - I had a Scottish Dance band for many years, and I know exactly what he meant. It doesn't always happen, but when you get the volume, tempo and lift right, the dancers ride on the wave and the whole thing just rolls along under its own momentum - it's hard to even make a mistake when playing (Ha!) under those circumstances, and it's a wonderful feeling to be part of something like that.
I remember one of the 'famous' musicians years ago talking on a broadcast about tempo. He likened the correct tempo to the speed of a horse doing a 'collected trot'. I had a lot of trouble with that, thinking of different speeds for different sizes of horses and so on, but eventually I got the hang of what he meant. When we play for dancing, the timing is actually related to gravity. When folk are dancing, it's all about lifting off the floor and falling back under gravity (or at least changing the weight from one foot to the other).
Gravity is constant, so the rate of something falling is constant, irrespective of the size (think of a petite wee lady dancing with a great hulk of a man, in perfect timing) so it's all about the length of time it takes for something to fall back to ground from a given height. It's all happening very quickly in dancing, but I'm sure it could be measured accurately with the technology we have nowadays. We all lift off to approximately the same height when dancing, so we all hit the ground ready for the next step at the same time + or - a very small margin, and that sets the correct tempo for that dance in beats per minute or whatever.
I think the lift comes from emphasis placed on the music by the musician - the famous 'dunt' of the bellows to increase volume spoken of by Jimmy Shand, and I believe that might be the clue the musician gives the dancers about WHEN to start the next lift movement. Thinking about it for the first time this morning, the 'dunt' seems to occur fractionally before the dance movement.
Because of the physically small size and the extra bellows work needed, SMs and Gaelics are ideally suited to 'dunting' - as is the Ranco, which is actually not much bigger, although the keyboards are extended to full size at the side.
Reading that all back, it's a bit scientific and complicated for this time of the morning - maybe I need more coffee?