This exercise is a good choice when trying to keep mobile in a sedetary world. (It works for EVERYONE  EVERYWHERE - in your office, at work, on the bus, you name it!)

Imagine your bench/stool to have a face of a watch; your pelvis can be seen as a waterbowl with water in it. Become aware of the position in which you are, right now.

Now, tilt your pelvis forward to 12 o'clock (pour water out of a bowl). Now tilt back to 6 o'clock (fill the bowl with water). Now tipp it over to 3 o'clock - you sit on the right sit bone - and  then to 9 o'clock (sitting on the left) - now you feel how your sides are shortening. This is the range of motion of your pelvis plus your spine.

You can now start moving in circles clockwise or counter clockwise. Feel if the movement is jerky or smooth, try to eliminate jerks in the next round. You will have a favorite direction - make sure to move in the other direction, too. Allow the spine to follow the movement of the pelvis - slouch slightly and staighten back up.

During practice, one can move freely, but during a (chamber music/orchestra) concert?

Diminish the size of the movements, so they are so tiny that only YOU detect movement, but nobody else would see it. Tiny movements allow the tissue and cells to regenerate, while a fixed position diminishes blood flow and leads to the accumulation of cell debrie and metablolistic waste.

MOVEMENT IS LIFE! So keep moving, even while sitting four hours.