I’m wondering if the higher foot placement allows athletes to get a better stretch reflex out of the achilles. when the new, higher tailed,blocks came out many coaches and athletes complained that it did not allow the competitors to get this, thus reducing block power. This could be an attempt to mitigate the effect of the higher back of the block.
For examples of complaints see the comments made by Ato in his blog Re JD, and Telez’s interview on the Canadian coaching site.