i know it is nothing to do with drift , but mainly the skill have 2 usage , 1st one is as u mentioned ... downshifting . 2nd is i mentioned ...
how to explain ...
http://upload.wikimedia.org/wikipedia/commons/thumb/b/b7/Racing_line.svg/300px-Racing_line.svg.png
normally ,before the corner apex , if u downshift and brake until reach the apex point , your RPM will not at the highest rev which mean will not getting the max power of accelerate after the apex point ...
in heel and toe , before the corner , u downshift enough and still braking and this time u clucth and rev it to max RPM (this moment is very near the apex point and only using brake to maintain speed) ... so when you reach the apex point ... your RPM is at highest rev ...