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You're strapped into the kite, so even if he let go he's still going for a ride. The only way out is to release the safety strap, but when you're being whipped around like that if you haven't practiced it it's going to be tough.
In the high performance sailing world we can get trapeze harnesses that have a quick release pull handle for the hook so if you get it caught on the rigging as you capsize you won't drown if you can't get it untangled/loose. Do kite surfer harnesses have something similar?