Variable Elastic Shoe Inserts

Underneath the heel, a horizontal slot, about the width of a credit card and the depth of three, runs from heel to toe. An elastic insert attaches to a built-in tensioner in the shoe that locks the insert into the slot.

That's the concept; the details are still sketchy.

I'm thinking the insert would look rectangular, but with rounded ends. One end would have a reinforced hole that would accept the tensioner, the other end would be anchored in the heel, when locked into the slot. To tension the elastic band, the shoe would be bent from the toe upwards (foot not in shoe). The farther the shoe is bent, the more tension the band would sustain. The tensioner would then sustain tension until a special release mechanism was triggered (bending the shoe in half?).