Rails' flash vs flash.now

I've seen these two things on several projects and never knew the difference. Until now!

 Boiled down to the very basics, use flash.now before a render call, and flash before a redirect_to.

  flash persists across an action, so if you use flash for a render, the flash message will show up on the rendered page AND the next page after clicking a link. Conversely, if you run flash.now on a redirect_to, you won't even see the message because the flash.now doesn't persist across requests. 

Thanks to these resources:
http://trace.adityalesmana.com/2010/10/difference-between-flash-and-flash-now-in-ruby/
http://ionrails.com/2009/09/19/flashnotice-vs-flash-nownotice/