I've been trying this out to distract people less. Yesterday I took it to a treadmill. It was nearly silent compared to the thuds I'm used to hearing with each footstep. At first I found it easiest to achieve this effect by stepping pretty much only with the balls of my feet. Then I found that I could bring my foot down beginning with the balls but smoothly coming down further later in the step to achieve even greater silence. It made my calves sore. What are the physics of this phenomenon? The first thing I thought of was 'impedance match' (don't ask me why).