Close-up of Dye Capsule.
Red dye is contained in a soft compressible capsule that is connected by a hairline channel via a hard cylindrical tube to an outer compartment (not shown). Sustained pressure, as from a mounting animal, is required to express the contents which are then readily visible in a compartment glued to the tailhead of the cow. The entire pressured sensitive device is called a KaMar (trade name).
Drost M (1979)