What exactly is program trading?

**Using software to buy or sell many stocks at the same instant.** In Korea it is not a loose phrase — it is a **regulatory classification**. An order from a single account that touches **15 or more KOSPI 200 constituents simultaneously** must be reported as program trading, and the daily totals are published separately. The common misunderstanding is worth clearing first: **program trading is not 'an AI deciding what to buy'.** A human decides 'sell this basket now'; the software **splits and fires the orders at once**. It is a question of execution, not judgement.

What is the difference between arbitrage and non-arbitrage program trading?

**Whether futures are involved.** **Arbitrage trading** works the gap between KOSPI 200 **futures and the underlying cash basket**. When futures trade above the index (contango), the trader sells the expensive futures and buys the cheap basket; in backwardation, the reverse. At expiry the two converge and the gap becomes profit. **Non-arbitrage trading** ignores futures entirely: it builds a basket of 15 or more KOSPI 200 names and moves it wholesale — an index fund matching new weights, a pension fund executing an allocation change, a foreign fund cutting its Korea weighting. **Trillion-won days come from the second kind**, because arbitrage is limited by the size of the gap while a basket trade is limited only by the size of the decision.

Is this what a sidecar stops?

**Yes — the sidecar's target is program trading orders specifically.** When futures move beyond a set threshold and stay there for one minute, program trading orders lose effect for **five minutes**. That is the difference from a circuit breaker: **a circuit breaker halts the whole market; a sidecar halts only these orders.** Trading continues in everything else. Reading a sidecar as 'the market stopped' is simply wrong. This brief covers the thresholds in 'What a sidecar is — and how it differs from a circuit breaker' and 'Sidecar trigger conditions — 5 percent, one minute, five minutes.'

What program trading is — the pipe that moves trillions of won in one session

Program trading means using software to buy or sell many stocks simultaneously. In Korea the definition is precise: an order from a single account touching 15 or more KOSPI 200 constituents at once is classified as program trading and must be reported as such. It splits into arbitrage trading, which exploits the gap between index futures and the cash basket, and non-arbitrage trading, which moves a whole basket regardless of futures. It is how trillions of won can leave in a single session, and it is what the sidecar rule actually halts

On August 24, 2026, foreign investors sold a net 3.68 trillion won of KOSPI stock and institutions 1.29 trillion won.

That is roughly ₩5 trillion — about $3.6 billion — in one session.

No institution places that many orders name by name. There is a separate pipe for it.

1. A category, not a description

Program trading means using software to buy or sell many stocks at the same instant.

In Korea, that is a regulatory classification rather than a figure of speech.

Criterion	Rule
Universe	KOSPI 200 constituents
Count	15 or more
Method	ordered simultaneously from one account

Cross that line and the order must be reported as program trading. Daily totals are published separately as a result.

2. The most common misreading

The phrase "programs are trading" invites an error.

	Program trading	Algorithmic / automated trading
Who decides	A human	The program
What the software does	Fires a decided order simultaneously	Chooses timing and instruments
Regulatory category	15+ simultaneous names	not a defined category

In program trading, the software's job is execution.

The decision — "cut the Korea weighting by 5 percent" — is made by a person. Executing it means selling hundreds of KOSPI 200 names in proportion, at once, and no human hand can produce that simultaneity.

It is a problem of hands, not of judgement.

3. Two kinds, with different ceilings

	Arbitrage	Non-arbitrage
What it targets	the futures-cash gap	the basket weighting itself
Uses futures	always	no
Ceiling on size	as large as the gap	as large as the decision
Typical user	broker prop desks, hedge funds	index funds, pension funds, allocators

Arbitrage

KOSPI 200 exists as both a cash index and a futures contract. In theory they converge; in practice they drift apart.

Contango — futures above cash. Sell the expensive futures, buy the cheap basket.
Backwardation — futures below cash. Buy futures, sell the basket.

At expiry the two must meet, and whatever gap existed becomes profit.

The size of arbitrage is bounded by the size of the gap. No gap, no reason to trade.

Non-arbitrage

Futures are irrelevant here. A basket of 15 or more KOSPI 200 names is bought or sold wholesale.

An index fund rebalancing after a constituent change
A pension fund executing an allocation decision
A foreign fund changing its Korea weighting outright

There is no ceiling. If the decision is to cut five percent, five percent comes out, regardless of what the market can absorb.

Trillion-won days are almost always this.

4. Why it pushes the index down

Not only because the volume is large. Because it is simultaneous.

Sold name by name across a day, the market has time to absorb it. When hundreds of names hit the offer at the same moment, the resting bids in each order book are consumed at the same moment.

Then this happens:

Selling eats through the bids in each name, step by step
Clearing prices step down
The index falls
Other participants see the index fall and sell

This brief covered a live example in "KOSPI closes at 6,696.96 on August 24, 2026," where retail investors bought a net ₩3.32 trillion and the index still fell 3.12 percent. Having a buyer does not set the price; the price is set where the seller walked it to — and simultaneity is what makes the walk fast.

5. Which is why the sidecar exists

	Sidecar	Circuit breaker
What it halts	program trading orders only	all trading
Market state	still open	suspended
Trigger	futures move sustained for one minute	index decline thresholds
Duration	five minutes	varies by stage

The sidecar is a device aimed at one order type.

Its logic is straightforward: break, for five minutes, the chain that runs from futures into the cash market. Five minutes is enough for remaining participants to look again.

So a sidecar trigger does not mean "the market stopped." The market keeps trading; only orders touching 15 or more names at once stop being accepted, for five minutes.

This brief covered the thresholds in "What a sidecar is — and how it differs from a circuit breaker" and "Sidecar trigger conditions — 5 percent, one minute, five minutes."

6. Two traps when reading the daily numbers

Trap	Reality
"Program selling drove the index down"	The program is an execution tool. A person decided to sell
"Program net selling = foreign net selling"	Different statistics. Investor-category flows and program-trading totals are separate series

The second is the one that gets mixed up most often. The ₩3.68 trillion of foreign net selling on August 24 is an investor-category figure, not a program-trading figure. The two overlap; they are not the same number.

This brief covered that family of discrepancies in "What foreign net buying is — and why the same day's number differs by publication."

7. In short

Program trading is 15 or more KOSPI 200 names ordered simultaneously from one account. A regulatory category.
The software does not decide. A person decides; the software executes simultaneously.
Arbitrage works the futures-cash gap and is bounded by that gap.
Non-arbitrage moves a basket wholesale and has no such ceiling. Trillion-won flows are usually this.
What moves the index is simultaneity more than size.
A sidecar halts only these orders, for five minutes. A circuit breaker halts everything.

8. What we could not confirm

August 24's program share — how much of that day's fall was program trading.
The rule text — the 15-name threshold is widely cited but not checked against the current regulation.
Current volume split — arbitrage versus non-arbitrage shares over time.
Cost factors — transaction costs, dividends and financing are omitted from the arbitrage description.

What program trading is — the pipe that moves trillions of won in one session

The three lines

Key questions