Basically if anything except feet or hands touch the ground due to contact with a defender, you're down. Shin, elbow, head, knee (of course). Pretty sure ankle, forearm, and wrist count too.
When I saw the Royal play, the first thing I thought was forward progress. I thought it was a mistake not to challenge, even though it might have ended up costing us a timeout. One of many mistakes by the team today. Hope Edwards is alright. Go Bills!