Hyper-extension of the knee is the knee joint moving past its normal range of motion. The ligaments, tendons, and other tissues that make up the joint are supposed to keep the joint stable to prevent damage. If those tendons and ligaments are too loose you could be a serious risk for an injury such as a dislocation, or ACL tear if you land wrong when tumbling.
In the short term, a knee brace could limit the excess range of motion and protect your knee joint. But the best and most reliable way to protect your knees is improve the overall stabilization of your knee joints by strengthening the muscles of the leg.
In particular, since the quads extend (straighten) your knee, I would suspect that you need to strengthen your hamstrings which flex (bend) your knee. I would also recommend strengthening your lower leg muscles including your calves as well as your anterior tibialis (the muscles that flex your foot)
These are just my personal suggestions from my experience with exercise science and years of coaching. To get the best results, your should seek out the advice of a licensed medical professional such as a physical therapist so that you can prevent injuries before they occur!
Best regards,
Coach Matt