The complement of on in a prepositional phrase is frequently a surface:
The vase is on the table.
But the complement of the preposition on can also be a body-part, the part which is contacting the surface and is supporting the body:
She stood on one foot.
She stood on her head. difficult, but not impossible :)
She lay on her side.
She lay on her back.
She lay on her stomach.
She was on her knees, looking for her door key in the grass.
She was on all fours, looking for her door key in the grass.
Sometimes on has the meaning of onto.
She rolled over onto her back.
She rolled over on her back.
That is, she assumed a position such that her back was contacting the surface below (the grass, the floor, the bed, the lounge-chair, whatever).