according to some documentation that I have, they use 50ft leads then attach weight, letting the line out according to achieve the depth required for the speed that your travelling with the weight chosen to use.
Not that complicated , lol
then some more food for thought, once a weight is deployed, being trolled, it will only achieve a certain depth and start to flatline, for eg a 4oz weight, going 2mph would get to 30,32 feet and start to level out, bait dependant as a diving crank will help pull it down more.
Enjoy the catching