1. Why does the line tangle when the lure stops?
In a baitcasting reel the spool turns as the line leaves. The flying lure pulls the line out, and the line makes the spool spin. This setup is good for accurate casts, but it has a built-in catch [1].
Early in a cast the lure is fast and pulls hard, so the spool spins fast. Then the lure slows down in the air, or hits the water, and the pull on the line drops. The spool, though, is a spinning object, and spinning objects tend to keep spinning (this is called inertia). So the spool keeps paying out line faster than the lure is pulling it away. The extra line has nowhere to go and piles up loose on the spool, where it knots. That pile-up is backlash.
Patents for these reels describe it in much the same way: a baitcasting reel's spool turns during the cast, so it is more prone to tangling than reels whose spools stay still [2]. This is why nearly every baitcasting reel has a brake, and why anglers also press the spool lightly with a thumb to slow it down.
2. How does a centrifugal brake slow a spinning spool?
A centrifugal brake uses small weighted levers or shoes mounted on the spool. When the spool spins, centrifugal force pushes them outward until they rub against a ring fixed to the reel body. A patent for such a device describes brake levers that swing toward the ring "under a function of centrifugal force produced by rotation of the spool" [1].
A simple way to picture it: as the spool spins faster, centrifugal force moves the weights outward, and in general the braking force grows with spool speed, so the brake pushes back most when the spool is going fastest, which is when backlash is most likely. As the spool slows, the push fades. One patent's background notes that the braking force of this type rises with spool speed [2].
It has trade-offs. Because the parts rub, they wear, and that rubbing can also shorten the cast a little, as the patents themselves note [2].
3. How can a magnet slow the spool without touching it?
The other common design uses a magnet. A patent for a magnet-based anti-backlash device describes a plate that moves toward a permanent magnet in the spool cover as the spool speeds up, so the spool is "smoothly and precisely braked" [2]. Because this design brakes through magnetic force rather than friction at the spool, it avoids wear at that braking point.
Here is a way to picture why a magnet can slow a moving metal part: a conductor such as metal moving through a magnetic field can feel a drag, and its strength depends on the material, the speed and the magnet's strength. Moving the magnet nearer or farther is how many reels let the angler set the brake strength with a dial. In many designs, a magnetic brake can act over a wider range of spool speeds, while a centrifugal brake is strongest when the spool is fast [2]. Makers also combine ideas, and newer reels use electronics (see the note on electronic brakes).
4. Where did the baitcasting reel come from, and why is it still hard to cast?
Reels with a turning spool are old in the United States. George W. Snyder, a watchmaker and silversmith in Paris, Kentucky, is credited with making the first American-made fishing reel, in 1820, and with a first reel for his own use around 1810. His design came to be called the "Kentucky reel". He had no patent, and other craftspeople copied it. The Wikipedia account suggests that their skill at cutting small gears made watchmakers well suited to the work [3]. This account rests mainly on one encyclopedia entry, so treat the exact dates as "reportedly".
A century and more later, the idea was refined by a company that began in watches too. Swedish firm AB Urfabriken, founded in 1921, made watches, telephone timers and taximeters before turning to fishing reels during the Second World War [4]. Its Ambassadeur baitcasting reel reached the United States in 1954, and a 1955 magazine advertisement introduced the models 5000 and 6000 [5]. Reel-history writers often point to this family when they discuss the centrifugal brake, but the sources I could read in full do not give the year the brake was introduced, so this note does not claim one.
Brakes have made baitcasting much easier, but none removes backlash completely. A heavy lure and a light one need different brake settings, and so does a headwind. That is why beginners are often told to start with the brake set strong and weaken it as their thumb control improves.
5. How can you see an overspinning spool for yourself?
You do not need a fishing reel. Slide a spool of sewing thread onto a pencil and hold the pencil level with both hands, or ask an adult to hold it. Pull the loose end of the thread sharply, so the spool spins and thread unwinds. Then stop pulling. The spool keeps turning for a moment and puffs out extra thread that was never pulled, a tiny backlash. Try a gentle fingertip on the spool and see how much thread it saves.
If you visit a tackle shop with an adult, ask to see the brake dial on a baitcasting reel, and what turning it changes. If you want one thing to jot down: "Backlash happens because the spool keeps spinning after the line stops pulling."