I've noticed most lifters who appear fast under the bar (Dimas, Ilin, Cal Strength guys, Catalyst lifters, etc...) go on their toes during the double knee bend and remain on their toes as the legs re-straighten. For example, look at 0:57 - 0:58 on this video:
I had thought you should only come on your toes after the legs have re-straightened, as an after effect of the force of the second pull. Is this incorrect? To me, it appears as though they are driving through their toes, but I have heard this is incorrect. Are these lifters doing this on purpose or is it an artifact of something else (i.e. aggressive pull under)?