Proos Electric Downrigger Question

[DEFIANCE1]

I just picked up a set of 4 electric Proos downriggers, three of them have cable with a small blue wire that runs throughout the cable, the cable then connects to the downrigger spool, and the wire to a small brass knob looking thing on the spool. Anyone know what this cable is for? Down temp or maybe short stop feature? Any info would be appreciated thank you.

[L&M]

Fish Mate down temp cable. The cable has a thermister woven in to it at the ball end. Without the head unit to it the cable is pretty much usless to ya.

[DEFIANCE1]

I did some research on that unit and it does not look like the unit is even available anymore. Atleast I know what it is for now, thank you.