Opposing Jammer has got lead (oh noes!) and your jammer is stuck behind a wall still on their initial pass. You are in front of the wall. You turn around (skating backwards), reach through the wall, and grab your jammer. Your jammer goes limp. You drag them forwards, through the wall.
You can not block a downed skater. You can assist a downed team-mate. No-pass/no-penalty on the entire pack for your jammer and they're out!