\uproot - used to fine-tune the placement of the index inside \sqrt or \root.
{ \sqrt[... \uproot #1 ...]{...} \root ... \uproot #1 ... \of {...}}
\uproot command fine-tunes the placement of the index inside \sqrt or \root where the argument is a small integer: a positive integer moves the index up;a negative integer moves the index down.
\sqrt[3]{x}
\sqrt[3\uproot2]{x}
\root 3 \of x
\root 3\uproot{-2} \of x