The most important part of FINVSQRT is the right shift by one bit. This halves the exponent, which gives the square root to within a factor of 2. Subtraction inverts the exponent since we're looking for an inverse square root. The second most important part is the Newton-Raphson iterations which refine the initial approximation. The most magical seeming part is also the least important: what you do with the mantissa in the initial approximation.
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The most important part of FINVSQRT is the right shift by one bit. This halves the exponent, which gives the square root to within a factor of 2. Subtraction inverts the exponent since we're looking for an inverse square root. The second most important part is the Newton-Raphson iterations which refine the initial approximation. The most magical seeming part is also the least important: what you do with the mantissa in the initial approximation.