The Silmari

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

⑤ After-Hours 18:002026.08.21 Fri · No. 18Sentiment gauge: Neutral (composite score 0.9) — 20-day volatility at 5.97% (93rd percentile, confirmed today); leverage-ETF share at 12.71% (95th percentile, confirmed today); credit-balance and short-surge inputs are on a publication lag and reflect Aug 20 / Aug 18 data respectively.

⑤ After-Hours · published 18:00 KST · after the regular session

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 15:30 KST

verdict at evening

KOSPI +0.88% — Samsung Electronics leads; KOSDAQ slides 4.63%

The KOSPI closed Friday, August 21 (2026) at 6,912.95, up 0.88% on the day, as Samsung Electronics and SK hynix rallied while most other sectors retreated — a split session that left the index holding its advance almost entirely on semiconductor weight.The KOSDAQ shed 4.63% to close at 801.94, with broad selling across biotech, power equipment, and defense names; retail investors provided the only net buying on the junior board today, absorbing ₩623.9bn in net purchases as institutions and foreigners sold.

Key points

01KOSPI closed at 6,912.95 (+0.88%); intraday range was 211.68 pts (3.14% of index). Samsung Electronics +3.87% and SK hynix +2.31% were the primary drivers; 11 of 12 tracked sectors closed lower.

02KOSDAQ fell 4.63% to 801.94 — the morning call of –0.5~+0.5% missed by a wide margin. Defense & aerospace (–7.4%), construction (–5.36%), and power & electrical (–4.42%) led sector declines on both boards.

03On KOSPI, foreigners net sold ₩176.0bn while institutions net bought ₩248.1bn (provisional, collected 17:52 KST); retail investors net sold ₩1.165tn. On KOSDAQ, all three professional groups sold while retail bought ₩623.9bn.

04The collapse-precursor score stands at 3 out of a possible maximum (ADRAdvance-Decline RatioThe ratio of advancing stocks to declining stocks on a given exchange on a given day; readings well below 1.0 indicate broad market weakness even if the index holds up.Today’s ADR signal is one of two active inputs in the collapse-precursor scoring model, reflecting divergence between index-level gains (driven by large caps) and deteriorating breadth. signal active, short-selling-surge signal active). In the current regime (2026–), the 5-day return after a short-selling surge averaged –0.15% with a 49% down-probability across 120 events.

02 · NXTNextrade Alternative ExchangeKorea’s first alternative trading system (ATS), launched in 2026, which operates pre-market, main, and after-hours sessions alongside the KRX main board.NXT after-hours data provides an early read on sentiment between the 3:30 pm KRX close and the next morning’s open, though volumes and prices differ from KRX and figures carry a 20-minute delay.NXT intraday — 20-minute delay · as of 19:25 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-hours snapshot: 523 decliners vs 74 advancers among 608 names, mirroring the main-session skew

As of the 20-minute-delayed snapshot at 17:25 KST, NXT (the alternative exchange, Nextrade) — figures below are NXT-only and are not KRX main-session data — showed 523 decliners against 74 advancers across 608 listed names, with combined turnover of ₩194,708bn on NXT alone; these are not today’s confirmed figures (confirmed NXT settlement data is registered in the evening).Samsung Electronics led NXT turnover at ₩81,489bn (NXT, 20-min delayed, +0.18%), followed by SK hynix at ₩51,085bn (+1.66%); Samsung Electro-Mechanics (–6.45%), SK Square (–1.87%), and Doosan Enerbility (–3.56%) rounded out the top-five by volume, all in the red — consistent with the day’s broad sectoral weakness.

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

S&P 500 futures –0.97% and Nasdaq 100 futures –1.20% overnight signal a gap-risk setup into Monday’s open

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 19:53 KST on 2026-08-21

Evening Trading Principles

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

Collapse-precursor score at 3 with both ADR and short-surge signals lit — volatility at the 93rd percentile warrants sizing discipline into the weekend

Today’s collapse-precursor score of 3 reflects two active signals — ADR and short-selling surge — against a backdrop of 20-day realized volatility at 5.97% (93rd percentile) and leverage-ETF share at 12.71% (95th percentile); when multiple signals overlap on a Friday, position sizing that accounts for a potentially wider Monday opening gap is a standard risk-management consideration.In the current regime (2026–), the 5-day return following a short-selling surge averaged –0.15% with a 49% down-probability across 120 events — the distribution is close to symmetric, which means neither a directional bet nor outright panic is supported by the base rate alone.The credit-balance and margin data reflect the prior business day (Aug 20) due to a T+1 publication lag from the Korea Financial Investment Association; readers who rely on those figures for overnight risk assessment should note that today’s final readings will not be available until Monday.