Softness is a three dimensional phenomenon. When you compress a material it squeezes out sideways. Typically it pushes out about a third the amount of compression. This number is called Poisson's ratio. This is the beginning of the story, not the end. Try holding a pan scourer, one of those little blocks of sponge, between the palms of your hands as though you were clapping. Squeeze and it compresses. You knew that. Now try bringing your little fingers together without moving your thumbs. It resists being squeezed, but does very little to keep your palms parallel. Now try a shearing action, as though you were rolling a piece of plasticene between you hands. You encouter a little more resistance than you did when squeezing, you will need to squeeze a little to stop it sliding. Now try a twisting action, by pointing the fingers of one hand down and the fingers of other hand up. Again you will need to squeeze a little to stop the sponge from sliding. The softness that cushions your clapping to silence has brought with it flexibility to five other motions. A nice, soft shoe heel would wobble all over the place and be too squishy to walk on.