Clarification of "Instant Hotspot" behaviour

Hopefully someone can help me with how the Instant Hotspot feature is supposed to work between an iPhone and iPad.

I need to know whether the iPad is supposed to rejoin the iPhone hotspot automatically after waking from sleep etc. I seem to have to mess around selecting the wifi network manually each time - a long way from instant!

It seems pretty sketchy, but the documentation isn't clear on this point - ideally I'd like it to join if a known wifi network isn't available.