Meaning: get busy/work harder (example 1, example 2)
Rewrite 1: fist press
Rewrite 2: full body face press
Rewrite 3: full court grapple
Rewrite 4: muscle pin it
Rewrite 5: gang tackle it
Comment: I think in this instance, "knuckle down" means to bear down upon or bear your weight on something. I've attempted to wrestle this one with as many grappling metaphors as I could manhandle without getting sweaty and pulling a sciatic nerve.
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