Where is this in the NFHS Rule Book?

[jmkupka]

we've all done games where the powder dirt is so soft and deep in the batter's box that the ball just goes down and dies there.

In the batter's box. In fair territory.
Batter-runner (that's what she is now) then realizes she has to run, and touches the ball on the way out.
There's no rule that protects her from contacting a fair batted ball just because she has one, two, or three feet still in the box.

This is not a case of the batter hitting the ball into herself. We know that when we see it. Even if she chips it down in front of the plate and it caroms into her leg (in that little area of fair ground). We'd kill that immediately.

But if she makes an act of a batter-runner, and runs into a ball in fair ground, much like Irish's description of "an act of interference", she's out regardless of where her feet are.