Reel drag
The drag gives line away once the pull exceeds what you set. It does not stop the fish and it does not save you from one — it turns a lunge into a steady pull. While the spool slips, the load on the tackle stops rising, and nothing breaks.
It is set to the tackle, not to the fish
«How tight should the drag be for carp» is the wrong question. The drag is set to the weakest link in your own tackle: the fish does not care where things give way, and they always give way in the same place.
That place is almost never the line itself. No knot holds as much as the unknotted line, so the knot sets the ceiling. The drag has to slip below that ceiling — which is why the same reel with the same line is set differently depending on how the rig is tied.
Front and rear is a location, not a principle
The friction washers are the same in both cases and work the same way. What differs is where the adjustment knob sits: a front drag is turned on the spool itself, a rear drag behind the body.
The front one sits closer to the washers and adjusts more finely, but you have to reach past the rotor to get to it. The rear one is easy to tweak once the fish is on and your hand is already on the body. Both live on spinning reels; on centrepin reels a ratchet check often replaces the drag altogether, and that is a different mechanism.
How it is checked
Not on the water and not by eye. Run the line through the guides, tie it to something solid and pull with the rod the way a fish pulls — by the blank, not by hauling the line with your hand. The drag should begin to give line smoothly, without a jolt at the start.
A jolt at the start is a fault of its own: the washers have stuck together. Such a reel holds until it breaks free and then gives line far too easily, and it is at its most dangerous in the first second of the take.
The baitrunner: a second drag
Some spinning reels carry a second, very light drag switched in by a lever. It is there for rods left in a rest unattended: the fish takes the bait and pulls line almost freely instead of dragging the rod into the water.
One flick of the lever brings back the main drag, set beforehand. It is not a third mode but a switch between two values you have already dialled in — and both are set before fishing, not during a take.
When the drag cannot help
It only works while there is line to give. At close range — the fish under your feet, a few metres off the spool — there is no stretch left, and the lunge reaches the tackle almost undiminished. What saves you there is your hand, not the drag: you drop the rod tip after the fish.
The second limit is braided line. It barely stretches, so there is nothing to absorb the lunge except the drag and the blank. With monofilament the line itself takes on part of that work.
What we do not state here
How much the drag should give as a fraction of the breaking strain. Such figures circulate on forums, but we found no verified source for them, and an invented number in a reference is worse than no number at all. The check that works by hand is described above.