The Commentary eclucidates this:
Nisidati pallankam abhujitva ujum kayam panidhaya parimukham satim upatthapetva so satova assasati sato passasati = "Sits down, bends in his legs crosswise on his lap, keeps is body erect, and arouses mindfulness in the object of meditation, namely, the breath which is in front of him. Mindful he breathes in, and mindful he breathes out."
"Bends in his legs crosswise on his lap." Three things pertaining to the sitting posture of the yogi are pointed out by that: firmness of the posture; easefulness of breathing due to the posture; and the expediency of the posture for laying hold of the subject of meditation.
One sits in this posture having locked in the legs. It is the entirely thigh-bound sitting posture, and is known as the lotus, and the immovable posture too.
"Keeps his body erect." Keeps the vertebrae in such a position that every segment of the backbone is said to be placed upright, and end to end throughout. The body, waist upwards, is held straight.
"Arouses mindfulness in front." Fixes the attention by directing it towards the breath which is in front.
"Mindful he breathes in and mindful he breathes out." Breathes in and out without abandoning mindfulness.http://www.accesstoinsight.org/lib/auth ... wayof.html