Rock has two options for ground throws. One throw tosses you back into the air and resets the ground loop, and one throw just does a normal ground throw for damage.
To break the ground throw loop one, push (or mash) B when you are grounded, and your head is towards him, and push A when your feet is towards him.
Also a sidenote: In some stun setups he will attempt a low throw that will catch you in the middle of your ukemi. You can also try to break those throws too (like any other throw), but if you get in the ground throw loop, refer to the instructions above.
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