Here's a simple picture with minimal reading required.

This is very different to the technology used in the free/wait-list Beeper Cloud app and all the other previous attempts at an iMessage for Android app.
To summarize:
All messages are sent directly between your device and Apple's servers. You do not even need an AppleID. There is a cloud server involved but it's only job is to send push notifications to Android so they app knows when to download new messages (securely with iMessage encryption) from Apple's servers.
No message contents are sent through the cloud server, it just notifies your device when there are new messages. This is necessary because Apple servers obviously do not support Android push notifications.