This protection is seen on many high-profile releases by Epyx from 1985-1987. Summer Games II, Winter Games, World Games, Rescue on Fractalus, Barbie, G.I. Joe, Hot Wheels, etc.
The protection was written by Scott Nelson.
The copy protection is checked on track 20. Starting from sector 1, the number of bytes between Syncs are counted for the next 17 blocks. Also, the first byte triggering the sync is read (in other terms, the last byte until the sync signal is registered). If any of these values don't match the expected values, the copy protection failed.
|length||last byte b4 sync|
|Header sector 1||$11||$af|
|Data sector 1:||$4d||$ff|
|Header sector 2:||$11||$af|
|Data sector 2:||$4d||$ff|