Reverse mirror:
ABC carries a regional slate of games in the 3:30 ET time slot. Generally one game from each of the True Major conferences which ABC has rights to (ACC, Big 10, Big 12, Pac 10). Each ABC station carries the game of local interest.
Now ESPN will carry two games. The Big 10 game on 206 (in HD) and one of the other 3 on 210 (in SD only). If ABC in an area is carrying the Big 10 game, then 206 will be blacked out and 210 opened up. If ABC is carrying one of the other 3, then 210 will be blacked out and 206 will be open, with the Big 10 game.
This is part of the Big 10 contract, insuring national access to the Big 10's #1 game of the week.
Hopefully ESPN will keep the HD feed for this reverse mirroring. Since I often get the Big 10 game on ABC, I often have to watch the other game on 210 in SD.