Its part of the Ball's Patented Crown Protection System, making sure the crown is properly screwed in before diving to prevent possible water ingress.
Extracted from Ball website
A crown protection developed for the Engineer Hydrocarbon series
A special crown protection system was designed for the Engineer Hydrocarbon series to guarantee its exceptional water resistance. A protective plate is placed around the crown, which ensures the crown must be screwed-in to its original secure position after time adjustment. The shock resistance test conducted on Engineer Hydrocarbon models also tests the crown protector to ensure it will prevent damage to the crown and resist water leakage as well. The crown is tested with external shocks to ensure its durability up to 7,500Gs.
