What "hechi" means, and which fish the anglers are after
In Japan, hechi is not a fish. It is the narrow strip of water right against a quay or pier wall. Anglers use the word for the fishing as well, calling it hechi-zuri. The style is said to have started in Tokyo Bay and is popular around Yokohama [1].
The target is the black sea bream, called kurodai in eastern Japan and chinu in the west (scientific name Acanthopagrus schlegelii). It lives in the western Pacific, in sheltered bays, on shallow rocky reefs and in brackish water, and it is not an endangered species. Wikipedia gives the maximum size as about 50 cm [3], and a Japanese angling writer describes hechi catches of up to around 50 cm [2].
Why black sea bream gather along the wall: a hidden pantry of shellfish and crabs
Below the waterline, a harbor wall is not bare. Japanese anglers report a layer of mussels near the high-tide line, with crabs, worms and shrimp hiding in it [1], and they also scrape barnacles from walls [2]. In effect, the wall is a vertical reef that people built by accident.
The black sea bream is well equipped to eat from it. It is a predator that feeds on molluscs and bristle worms (polychaetes) [3], and anglers report that it also eats crabs and other small animals [1]. It lives in brackish water, where river and sea mix, so it can cope with the muddy water of a river mouth in the middle of a city [3]. A place that looks barren to us is, to this fish, a restaurant with plenty of hard-shelled food. Anglers say you can sometimes see the fish nibbling at the wall near river mouths [2].
This is why the bait matters. Anglers in Japan commonly use small shore crabs, or mussels scraped from the same wall [1]. In other words, they offer the fish exactly what is already growing there.
How the fishing works: a bait that seems to fall off the wall
The gear is simple. A line, a hook and a few tiny split-shot weights are all that is tied on, and the rod and reel are plain, too [1][2]. A beginner's guide says the whole outfit can be very cheap compared with other styles of sea fishing [2].
The skill is in how the bait goes down. The angler lowers it right against the wall and lets it sink on a line that is neither pulled tight nor left slack. The aim is to make it look like a crab or mussel that has just come loose and is tumbling down the wall [1]. The guide explains that these fish react well to things falling past them, so a falling meal is what the angler tries to imitate.
Then comes the hard part: noticing the bite. With so little weight on the line, a sea bream that takes the bait changes how fast the line sinks, and the angler watches for that change, or feels it through the fingers, instead of waiting for the rod tip to jump [1]. It looks like standing still and doing nothing. It is closer to listening very carefully.
Is the fish at the end of the line a male or a female? It can depend on its age
The fish on the line has a surprise of its own. The black sea bream is a "protandrous hermaphrodite," which means young fish are mostly male and some later become female. Researchers who studied its gonads found that testicular tissue dominates in young fish, while ovarian tissue grows alongside it. In some 3-year-old fish, the testicular tissue shrinks and eggs develop, which is the sex change [4]. Not every fish changes at the same age, and not every fish changes at all, so a big one is not always a female [3][4].
How to look at a harbor wall like an angler, without any fishing
You do not need a rod to test this idea. If there is a harbor, a canal or a river mouth near you, ask an adult to go with you and look at the wall from a safe spot at low tide. Look for dark mussels and white barnacles, and for the line where they stop. With the adult, compare the same wall at different tides if you can, always from a safe spot. That band is the pantry.
Safety note: Water edges are dangerous. Go with an adult, stay back from the edge, and never lean out over the water to see better. Do not scrape animals off a wall unless an adult says it is allowed.
If you want to know more about the gear and the timing, the Japanese angling guides listed below go into much more detail than this page.