The three-dimensional area downwind of you in which the kite can fly. Where it sits inside that area determines the pull.
Picture a quarter sphere with you at its centre, opening up downwind. At the edge of that sphere — just above the water left and right, and at the zenith directly overhead at twelve o'clock — the kite pulls least. Low and central, straight downwind, lies the power zone, where it generates many times the pull. That's exactly why you steer the kite through that zone for a water start, and exactly why an uncontrolled dive into it is the leading cause of kite accidents on land. Note: twelve o'clock is a parking position, not a safe place to sit in gusts — better to keep the kite around 45 degrees and work it actively.