It sets a variable, SBOX_REDIRECT_IGNORE for /usr/bin/tool, and then ensures that /usr/bin is at the head of your PATH.

Scratchbox uses host tools as much as possible (for speed), and also stuff within the devkit. However, the above allows you to override it.

So, for example, we could have a newer version of bison in Extras - your Build-Depends will specify the exact version - and override the version shipped as standard.