By making Samyama on the relation between the Akasha and the body and becoming light as cotton-wool etc., through meditation on them, the Yogi goes through the skies.
In plain words: By focusing deeply on how the body is linked to Akasha (space), and meditating on things as light as cotton, the yogi becomes light and can travel through the sky.
This Akasha is the material of this body; it is only Akasha in a certain form that has become the body. If the Yogi makes a Samyama on this Akasha material of his body, it acquires the lightness of Akasha, and he can go anywhere through the air. So in the other case also.
Namaste.
A calm guide to life, the mind and the old teachings.