Is not really autoroll,in the way described.
The receiver itself doesn't have any provision in itself to find the new key.
It relies on a server that keeps a database and matches,if available, the request from the receiver and send it.
I am sure in some feeds can be an easy task,but can also be a headache with so many feeds using sane name,same sid,same pmt,same video pid,the server can simply send the wrong key.
Why someone finds out if tuning a channel or feed where the key has not ever been made public to see if it "rolls"