1. What is the thumb actually stopping?
On a baitcasting reel, the spool turns as line leaves. The flying lure pulls line off, and that pull spins the spool. A Daiwa representative quoted in a Texas Parks and Wildlife magazine guide put the problem like this: a tangle happens "when the speed of your spool is greater than the speed at which line is being pulled from the reel by the lure" [1].
When the lure slows, because of gravity, wind or the water, the spool tends to keep going. It is a spinning object, and without a force acting on it a spinning object keeps spinning (this is called inertia). Extra line spills out faster than the lure pulls it, piles up on the spool and knots. Anglers call the knot a "backlash" or a "bird's nest". Another note on this site follows the spool physics and the brakes in more detail. This one is about the hand.
The same Daiwa representative states the goal plainly: the spool should stop "at the same time or before the lure does" [1]. The thumb is how a person makes that happen.
2. Where does the thumb go, and how hard does it press?
Many baitcasting reels have a thumb bar, a button that frees the spool so it can spin. It sits where the thumb can reach it "without removing a hand from the handle" [2]. Before the cast, the thumb rests on the bar or the spool, and pressing the bar frees the spool so line can run out. That description of the order is my own reading, and the exact shape of the bar differs from reel to reel.
After that, the thumb stays close to the spool. Outdoor Life describes slowing the lure in flight by "gently feathering the spool with the thumb" [2]. "Feathering" means touching the spool lightly, so that it slows but does not lock. The sources I read agree on "gently" but give no pressure figures, so this note gives none either: how much to press is learned by feel.
A patent shows that reel makers expect this. Daiwa's US patent 4,575,024 says that with older reels the angler had to use the clutch and also thumb the spool flange (the rim of the spool) at the same time. Its design lets the tip of the thumb land "naturally" on a position where it can do both [4]. That is the patent's own claim about its design, not a test of how well it works in practice. Its Japanese priority date is April 13, 1983, and it was granted in the United States on March 11, 1986.
3. When in the cast does the thumb press?
There are two moments. The first is as the lure nears its target. The Daiwa representative advises "gently thumbing the spool as the lure nears your target" to slow the lure's flight [1]. The second is the landing itself: the spool should be stopped by the time the lure arrives, so no extra line is left to loosen. One simple way to see why the landing matters is that once the lure stops, for example on the ground, its pull on the line ends, while the spool may still have spin. On water the lure can keep moving after it lands, so this is a simplification. That reasoning is my reading of the sources, not a quotation.
The same guide offers a way to find out where the spool's own tension should be set. With the lure tied on and the rod level, you press the button and let the lure drop. Ideally "the lure should sink to the ground and as soon as it hits, the spool stops spinning" [1]. A heavier lure needs a different setting from a lighter one, so the test is repeated whenever the lure changes.
4. Is the thumb only for preventing tangles?
No. Outdoor Life notes that the same feathering can give "a feather-soft landing". Stopping the lure too abruptly makes a heavy "plunk" that can scare fish, so the aim is to slow the lure and bring it to "a not-so-sudden halt at the water's edge" [2]. By easing off or pressing more, the angler also changes where the lure comes down, for example to stop it from going too far.
I could not find a source that gives numbers for any of this, such as how many metres of distance a thumb adds or how many hours it takes to learn. These are descriptions from experienced anglers and a tackle maker, not measurements.
5. If the reel has a brake, why is the thumb still needed?
Reels do have brakes. Daiwa explains that its Magforce brake does not pull on the spool with a magnet. Instead, a non-magnetic metal cylinder turns with the spool between magnets, which creates swirling electric currents (eddy currents) that push back against the spin. Daiwa says the braking force is proportional to spool speed: spin too fast and the force is large, slow down and it weakens, adding "the last bit of stretch" to the cast. The company dates the idea to 1982 [3].
The catch is that brakes are a compromise. A stronger brake means fewer tangles but also shorter casts, and Bassmaster's Jason Christie tells anglers teaching kids or beginners to keep the tension on the tighter side and accept a shorter cast [5]. In the Texas guide, Daiwa's Bill Liston says a magnetic brake can help "educate your thumb", and another expert says that ideally "your thumb should control most of the spool's speed" [1]. That sets up a division of labour: the brake does the broad, automatic slowing, and the thumb does the fine adjustment. The brake does not make the thumb unnecessary, and the thumb does not make the brake unnecessary.
6. How can you feel this for yourself?
Put a roll of toilet paper on its holder, pull a length of paper with a quick tug, and let go. The roll keeps turning for a moment and the paper goes slack. Try again, and this time touch the roll with the pad of a finger just before it loosens. How little pressure stops it? Roll the paper back up afterwards. This shows only that spinning things are hard to stop, not what a real spool does.
If you visit a tackle shop with an adult, ask to see a baitcasting reel and find where the thumb rests. Keep fingers away from the handle and gears. Real casting involves lures with sharp hooks, so do it only at a place that allows fishing, away from other people, and with an adult. Outdoor Life suggests practising first with a gentle lob-style cast, and, for beginners, setting the brake tight and loosening it gradually [2].