Like Matt said, you bend at the waist...

Do this experiment, in your house...go get your bow, come to a full draw....with your arm straight out in front of you at full draw, twist left, twist right without moving your arm that' s squared to your body, now bend at the waist allowing for a pretend downward shot, bend back like you might shoot up without raising or lowing your arm....that should never happen...the form you have while standing on level ground should be the same, from the waist up at all times shooting up or shooting down....

The form you have while at full draw on level ground, from the waist up should never change when shooting down or shooting up.