The mechanics:

1. If you want to play challenge games, click the box that says "Enable Challenges" in the Challenge Play menu. If you do this, other players can challenge you and you can challenge them.
2. Note the rules at the top of the "Enable Challenges" page for some other mechanics.
3. You get more points in the rankings if you beat stronger teams, and fewer if you beat weaker teams. The same goes for losses: if you get upset by a weaker team, you take a bigger hit on your points and rankings.
4. Since challenges can take place in the offseason, we assume no injuries. If you want to model injuries, you can just take a player out of your lineup.

Let us know what you think! This is a new feature, so we're always willing to hear suggestions for improvements.