Warne also does the same thing with pitch.
Some insight to his rigorous methods is reflected in Pete Christlieb's notes to the Criss Cross disc called Conversations With Warne vol 1: "One day he played something very melodic and dissonant at the same time while offsetting or displacing the phrase in double time feel. 'How do you do that?' I asked him. He said 'You don't want to get involved, it will only confuse you.' He did however agree to write out a lesson plan for me to look at later.
The plan called for a phrase to be composed four bars long and memorized. You start the metronome at a reasonable tempo and begin playing your phrase an eighth of a beat later. Now start your phrase on the quarter and so on. Be sure to take plenty of change along with you to phone home. Designed to develop your mental dexterity, this exercise was set aside because it was, in fact, confusing to me at the time, I needed to be fluent and uninhibited."
This all sounds really cold, but Warne felt the more you know, the closer you can come to pure "expression of feelings" and he took great care to let you know "feelings" and "emotion" are distinct things.