Ye shall let nothing of it remain. — That there might be neither profanation nor superstitious use of what was left. (Comp. the requirement of the Church of England with respect to the Eucharistic elements.)
That which remaineth — i.e., the bones and such particles of flesh as necessarily adhered to them. These were to be at once totally consumed by fire. Thus only could they be, as it were, annihilated, and so secured from profanation.