The idea is to maintain the good "T" position with your shoulders while keeping the shoulders in alignment with the target. That's always true, whether you're shooting on level ground, uphill or downhill.

If you just elevate or depress the bow arm for shooting uphill or downhill, your shoulders are no longer aligned with the target. That means you won't hold as steady, your draw length will be shorter (which will move your anchor back an inch or more!) and you won't ever be able to keep proper back tension.

Instead of using your arms to point the bow at the target, you point your shoulders at the target and then aim the bow.

If you're not very high in the tree and the shot angle is fairly shallow, you might not have to bend at the waist at all. You can bend the front knee instead. That way you just lean the entire "T" toward the target.