Why do we have a person sit with head between knees when assisting them for fainting or shock if we can't lay them down?

Tha main cause of fainting is that, there is shortage of oxygen supply to the brain.

The blood is the one who carries oxygen and distribute to the different parts of the body.Because of gravity,the head of a person,during fainting, should be placed between the knees or below the heart.In this case, pumping of blood from the heart to the brain will be easy.