430EXII Slave question

[neuroanatomist]

I figured out what I was doing wrong, the builtin does flash but you are right it does not affect the photo.

Yep.

Note that the 580's master function is optical - you'll see the on-camera flash fire as a preflash which signals the slave flash(es), but the on-camera flash won't fire during the exposure.

That applies to the built-in as well.