The simplest explanation: Increases in intracranial pressure causes compression of the cerebral vessels , decreasing cerebral flow leading to ischemia and increase Pco2. This causes the reflex vasomotor center response in the brain to increase sympathetic outflow to the heart and blood vessels that increase mean arterial pressure which inturn increases cerebral blood flow.The reflex basically restores cerebral blood flow to an ischemic brain. Hope this helps.