Controlling the position of one's body and keeping a straight back are not contemplation, but can in fact become an obstacle to contemplation. ...when leaving the body 'uncontrolled' is spoken of, what is meant is simply allowing the body to remain in an authentic, uncorrected condition, in which it is not necessary to modify or improve anything. This is because, since all our attempts at correcting the body come from the reasoning mind, they are all false and artificial.

-- Chögyal Namkhai Norbu Rinpoche