Like running

[Sharla]

Changing from heels low and high i think works as a dynmamic sort of PNF stretch. The basis of PNF being if you allow a muscle to tire a little and then stretch to the point it can move voluntarily itself (without being forced by pulling on your leg etc) it allows an efficient functional increase in flexibility.

Increase in flexibility with normal PNF stretching is always good enough to be visible immediately and if you're not forcing your leg to stretch by using another limb to pull it you are working the range your leg can move in when you're using it.

So you work one stride type and those muscles get a little tired, then you change to the opposite which stretches some of the muscles you were just working (here primarily the quads and hip flexors i think) and it works the same way PNF stretching does. After a few rounds you've PNF stretched some muslces while exercising them.

Who knows the fact you're doing it with aerobic exercise and pumping all that oxygen through might have some added benefits too?