Pull jaws backwards. Decreases tension on throat. Also send sounds to a more open throat and preferably up through the top of the head (back of the throat)–to increase resonance and thereby cause less stress.
My personal experience though is that deliberately tightening the throat and presumably the vocal cords can create a higher shrill high, but this is more tiring on the throat. The above mentioned type of highs is more relaxing, but doesn’t sound as “tough”, although close.