tet spells and knee-chest

Can someone please explain to me why the knee-chest position helps when dealing with tet spells in cyanotic tetralogy of fallot patients?

What I've been told is that it tends to increase systemic resistance - thus decreasing the R->L septal shunt, and encouraging pulmonary flow.

I guess I don't see the physiology behind the knee-chest position increasing the systemic resistance?

(Does it increase venous return? Is it some form of reflex that triggers vasoconstriction? Is it increasing intrathoracic pressure (but that would decrease venous return) and thus increasing arterial tone similar to the mech behing valsalva?