When does auto-download get triggered with Podcasts app?

I just started using iPhone 5s (with iOS 7.0.2). I subscribed to a few podcasts and even though I have auto-download "Most Recent" and keep "All Unplayed", I was unable to figure out when the auto-download gets triggered.

Does it need Wi-Fi necessarily? (I have set the "allow cellular" option turned ON.) Does it have to be charging?

I realized that the download started a few _minutes_ after a new episode was discovered, and it was somewhat related to my entering a Wi-Fi hotspot area. I would like to know exactly what triggers it so that I can plan my commute (need new episodes downloaded before I start) :-)