Focus: Blocker awareness - being able to focus on other aspects of the game not just the blocking they are directly involved in. Jammer awareness - getting the jammers to practice checking with the bench coach.
Have everyone in the session form one big pack and send two or three jammers through the pack at a time. The blockers positionally block the jammers coming through the pack.
Have a bench coach standing to the side of the track signalling to the jammers, the jammers must imitate back to the bench coach what they are signalling to communicate that they have seen the benches signal.
Meanwhile the blockers are paying attention to a jammer who is sat in the penalty box. Every now and then that jammer will stand up, at this point the blockers yell 'jammer standing' when the blockers notice the jammer sits back down.
Run through until everyone has had a go at jamming.