This is the Youtube of Sports Science researching former Red Sox pitcher Bryce Florie's scary combacker accident from the 2000 season. It is pretty interesting.
A fastball coming in towards the plate at 95 mph comes back towards the pitcher at 120 mph. A pitch of this speed hitting a helmut has 2,400 pounds of force and not enough to cause a concussion. That same pitch batted back at the pitcher makes contact with a force of 5,500 pounds, or enough force to stop a Mini Cooper traveling at 10 mph dead in its tracks.
YouTube - Sport Science - Episode 8 - Bryce Florie