The Silmari

A daily data newsletter on the Korean stock market — silmari, ‘a clue: the loose end that unravels the knot’

⑤ After-Hours 18:002026.08.18 Tue · No. 15Neutral (score 0.64) — vol-20 percentile 93 confirmed; credit, margin, and short-surge inputs carry prior-session lags (last confirmed: Aug 14 / Aug 12).

⑤ After-Hours · published 18:00 KST · after the regular session · Note: flow figures are preliminary as published just after the close; the exchange’s confirmed figures appear in that day’s Final Edition (20:00 KST).

① Pre-Open07:30 ② Early Session10:00 ③ Midday12:00 ④ Closing16:00 ⑤ After-Hours18:00 ⑥ Final20:00

01 · VerdictRegular session confirmed (2-source reconciliation)

verdict at evening

KOSPI −1.55%, KOSDAQ −3.52% — Broad Selloff Defies Tuesday’s Historical Bias

The KOSPI closed Tuesday, August 18 (2026) at 6,869.83, down 1.55%, while the KOSDAQ shed 3.52% to finish at 834.20 — both markets moving against a Tuesday session that, in the 2026 regime through today, has shown a 71% up-rate and average gain of +0.23% across 31 observations.The KOSPI traded in a 427.84-point intraday range (6.3% high-to-low), touching its session high of 7,216.62 at 09:39 before a sustained slide carried it to a low of 6,788.78 at 15:08; this morning’s call — which projected a +0.5 to +1.5% gain for the KOSPI and flat-to-modest upside for the KOSDAQ — missed on both counts.

Key points

01KOSPI closed at 6,869.83 (−1.55%); KOSDAQ at 834.20 (−3.52%). Both morning calls missed — the models projected gains on both markets.

02Today’s KOSPI flow (provisional): retail investors net bought ₩741.3bn; institutional investors net sold ₩795.1bn (securities firms leading at −₩669.1bn); foreigners net bought ₩91.4bn. KOSDAQ investor breakdown is not available from this data source.

03Non-arbitrage program selling on the KOSPI reached −₩1,088.7bn net today, the dominant mechanical pressure; the 30-year U.S. Treasury yield topped 5.31% overnight — a level not seen in 19 years per the event calendar — adding to the macro backdrop alongside the Nikkei 225’s −0.90% move at its midday snapshot.

04The pre-market crash-signal score stood at 3 of a possible composite (ADRAdvance/Decline RatioThe ratio of advancing stocks to declining stocks in a market on a given day; a reading above 1 means more stocks rose than fell.The crash-signal sub-score uses ADR as an input; a reading of 2 on the ADR sub-score today indicates breadth was sufficiently negative to trigger a signal flag. sub-score 2, short-surge sub-score 1); sentiment gauge held at 0.64 (‘Neutral’ stage), with the 20-day volatility percentile at 93 — among the components with confirmed today’s readings.

02 · NXTNextrade (Alternative Exchange)Korea’s alternative trading system launched in 2026, operating pre-market, main, and after-market sessions alongside the KRX main exchange.NXT after-market data offers an early read on how sentiment is holding past the 15:30 KST KRX close; today’s snapshot showed 529 decliners vs. 69 advancers.NXT intraday — 20-minute delay · as of 17:32 KST

Evening disclosures —Short sellingShort sellingSelling borrowed shares first, betting the price will fall.A rising balance means more money is betting on a decline.balance

A window on whether the regular session’s move carried into the evening alternative venue

NXT After-Market Snapshot: 529 Decliners vs. 69 Advancers as Selling Continues Past the Close

As of the 20-minute-delayed snapshot (reference time: 17:32 KST, August 18 (2026)), NXT — the alternative exchange, not KRX main market — showed 529 decliners against 69 advancers across 608 stocks, with combined NXT turnover of ₩20,221.7bn; these are not confirmed figures — final NXT session tallies are registered in the evening.The five heaviest-traded names on NXT in this snapshot were SK hynix (−0.91%, ₩5,748.2bn), Samsung Electronics (−3.83%, ₩5,149.1bn), Samsung Electro-Mechanics (−7.96%, ₩1,356.6bn), SK Square (−3.38%, ₩482.3bn), and LG Electronics (−4.65%, ₩454.7bn) — all in negative territory in the after-market, extending the main-session direction.

03 · Gap risksRisk → same condition in the past → outcome distribution

Tomorrow morninggap risks 4names

A window naming in advance what could shake tomorrow’s opening price

Overnight Macro Headwinds Intact: U.S. 30-Year Yield at 19-Year High, S&P 500 Futures Down 0.69% at Last Read

ADR

Warning score total: 3

signal 2 points

Short-interest surge

Warning score total: 3

signal 1 point

Selling acceleration

Warning score total: 3

signal 0 points

Sentiment 90+

Warning score total: 3

signal 0 points

05 · Principleas of 17:55 KST on 2026-08-18

Evening Trading Principles

A window recording the trading principles to hold to on the evening of a crash day

Crash-Signal Score at 3 and Vol Percentile at 93 — Elevated Caution Warranted Into Tomorrow’s Open

With today’s pre-market crash-signal score at 3 (ADR sub-score 2, short-surge sub-score 1) and the 20-day volatility percentile confirmed at 93, the risk environment remains elevated; in the 2026 regime (n=111), the five-day return after a short-surge event has averaged −0.50% with a 51% down-probability, against a regime baseline of +2.87% — a gap that reflects the drag these events carry in the current period.Overnight U.S. futures as of the last collected reading (05:00 KST) showed S&P 500 futures at −0.69% and Nasdaq 100 futures at −0.81%; the 30-year U.S. Treasury yield reached 5.31% — the highest in 19 years per the event calendar — which, alongside elevated domestic volatility, represents a cluster of concurrent inputs rather than a single identifiable driver.Positions sized without accounting for a high-volatility, multi-signal day — intraday range of 6.3% on the KOSPI — carry asymmetric gap risk into Wednesday’s open; the judgment is each reader’s own.