It's not certain that the word means sway. In ancient Hebrew there is a root Kh/f/tz II which means to harden or stiffen. The other explanation, to bend, comes from Arabic usage, which might mean he's so powerful that even when his tail is swift it can bend.
Sway is likely to be a mistranslation unless the meaning of the word has changed in 400 years. Does anyone have an OED?