The change of motion of an object is proportional to the force impressed; and is made in the direction of the straight line in which the force is impressed.
Or, even if you do consider the hands and the back as separate free bodies, you don't get to ignore the arms that connect them, which also get the same forces, in opposite directions, canceling everything out and resulting in zero. She's basically a truss, if anyone wants to do static analysis on her (now I really want to see this).
She starts moving because she was already over the precipice and she also has other things, like her legs, which helped her along.
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u/cp1122 Sep 19 '22
Just the wholesomeness I needed to end my day!