Ok, well, if I were to claim to be an expert on this topic, it would only be after having spent an hour or so reading about it this morning.
Here's my best guess at the situation:
RPC1 drives:
1. The player software asks the drive for the data on the disc, and the keys required to decrypt it.
2. The drive hands over the data and the decryption keys, without caring one bit about what type of disc it's currently chewing on.
3. The
player software decides whether or not to start decrypting the content and displaying it, based on whether the region of the disc matches the region of the drive (or possibly of the software itself -- I'm not sure) or not. This, of course, assumes that the player software is playing by the book (pun intended).
RPC2 drives, using "standard" playback software:
1. The player software asks the drive for the data on the disc, and the keys required to decrypt it.
2. The drive says, "here's the data, but you're not going to get the decryption keys, because I'm a Region 1 device, and the disc I'm currently chewing on is Region 4."
3. The player software gives up and dies a lonely death.
RPC2 drives, using liberated software, e.g. the kind that's easy to find on Linux systems:
1. The player software asks the drive for the data on the disc, and the keys required to decrypt it.
2. The drive says, "here's the data, but you're not going to get the decryption keys, because I'm a Region 1 device, and the disc I'm currently chewing on is Region 4."
3. The player software says, "I don't care, you jerk. I'm forceably decrypting the data you're passing along to me, so you can keep your stupid decryption keys."