The removable plug with coin slot provides access to a small switch that sets the pre-flash intensity.

Switch to the right (as shown) is for digital cameras with pre-flash. This pre-flash mode provides a longer pre-flash to reduce chance of overexposure.

Slide switch to the left for cameras that do not utilize a pre-flash system. This position produces a shorter (less intense) pre-flash with pre-flash cameras. This would provide a brighter main flash if desired.