USA Softball August 2017 Clarifications
 

So,
"Keep in mind if the swing and the leaving of the base are very close to the same time the ball reaches home plate, it would be a hard situation for an umpire to sell that they could tell the runner left before the ball reached home plate"
means give the runner the benefit if the doubt and don't call marginally precise things that are not the intent of the rule.

IOW, no "gotcha" rules.