Lead core is not something a novice trolling should attempt. Lead core is very effective, but only if you “work” it the right way. If you just drag it around it is more of a hassle than an advantage. Learn to troll first with mono or FireLine then advance to Lead core!
The first advantage of lead core is that it is a weighted line, so it can pull crank baits down into depths that the bait itself can not dive to. Obviously you would not want to tie lead core directly to the bait as it is a very heavy and thick line and would dampen the baits action. So, a leader is tied to the end of the lead core. If fishing structure this leader would be 15 feet of 10 pound FireLine, if fishing open water the leader would be 30 to 50 feet of Berkley 10 pound XT.
The second advantage of lead core is that you can change how deep the bait swims by changing your trolling speed. This is especially useful on sharp breaks, on bottoms that are irregular or in situations (like fishing over trees) where you need to raise and lower your bait. In these situations, you can speed up your troll to lift the bait. The faster you go, the less the lead core will dive and this will pull your crank bait up to a higher depth. Conversely if you slow down, the lead core will dive more efficiently and pull the bait deeper. By speeding up and slowing down you can raise and lower the bait, keeping it close to whatever structure you are trying to fish.