What is the stand_up_word (not wake_word) for?

The configuration files have both. I’ve peeked at the source,
but I’m not sure what the HotWordEngine associated with the
stand_up_word is used for.

It’s used by the naptime skill:

Tell Mycroft to sleep when you don’t want to be disturbed in any way.
This stops all calls to Speech to Text system, guaranteeing your voice won’t
be sent anywhere on an accidental activation.

When sleeping, Mycroft will only listen locally for the phrase “Hey Mycroft,
wake up”. Otherwise the system will be totally silent and won’t bother you.
On a Mark 1 this also dims the eyes.