Pulling up to center split (pancake) on silks is a PULL, a COMPRESS, and a PUSH, utilizing LEVERAGE. Depending on your individual mobility, certain aspects may require more strength.
You have to pull up enough and compress to get the weight of your upper body THROUGH your legs. But if you consider the pull up is more like a chest to bar, or even muscle up, than a regular pull up, it also requires a lot of shoulder internal rotation.
Prerequisites:
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flat pancake
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chest-to-bar pull up
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flying pancake (with blocks on calves)