Original Research Lead hypothesis, built on the textbook 'evaporating liquidity' finding (Nagel 2012, RFS: short-term reversal returns, i.e. the return to liquidity provision, are largest right after market-wide stress) applied to crypto alt perps. On a market-wide down-shock day, forced deleveraging and liquidation cascades hit some alts far harder than their normal BTC beta implies; that excess (idiosyncratic) drop is a liquidity-demand price concession, not news, and it is partly repaid over the following days. The strategy is flat until BTCUSDT.OKX prints a UTC daily close-to-close log return <= -shock_k x its trailing 60-day daily std (prior days only). At that close it ranks every eligible alt in a fixed 22-name OKX USDT-swap universe by its same-day RESIDUAL return r_alt - beta_alt x r_BTC (beta = OLS slope of daily log returns over the prior beta_lookback days, at least 60 valid days), buys the n_long most negative residuals equal-weight, and shorts BTCUSDT.OKX for beta-neutrality (BTC short notional = average beta of the bought names x total long notional). Everything closes on a fixed clock after hold_days daily bars. New triggers are ignored while a book is open, so events never overlap. Venue: OKX USDT perpetual swaps (3.6% of the corpus, under the BINANCE 49% concentration), long/short market-neutral (long_short bucket is 24%), event-driven flat-most-of-the-time design (lesson L224). Execution: orders are market orders at the signal bar's close; on this 24/7 venue that price is the next bar's open (use_equivalent for next_bar_open_fill, stated here). Universe is point in time: an alt is eligible at an event only if it has >= 90 days of OKX daily bars before the event (listed_since / universe_at); OKX keeps no delisted swaps, so residual survivorship bias remains and is stated as a risk. PRE-REGISTERED FIXED-RULE MEASUREMENT (OKX 1-DAY closes 2020-01 to 2026-10, shock_k 1.5, beta 90d, n_long 3, hold 5d, non-overlapping): 94 events; long weak-3 arm +2.04% mean 5-day return; BTC-beta-hedged book +1.82% per event on long notional (t ~1.97, hit rate 54%, worst event -17.9%), positive in 5 of 7 calendar years (2022 -0.3%, 2023 -1.2%). Per L230 the alternative short arm (shorting the 3 STRONGEST-residual alts) was measured too and is rejected: those names rose +1.44% over the same 5 days and the dollar-neutral alt-vs-alt spread had a -29% single event (a squeeze), so the hedge is BTC, not alt shorts. COST ARITHMETIC: per event the book pays ~0.10% RT on the alt legs plus ~0.10% x beta(~1.07) on the BTC leg, ~0.21% of long notional, vs +1.82% gross hedged capture, net ~1.6% per event (about 8x the cost). Per-trade average across the 4 positions per event (three alt legs ~+2.0%, BTC short leg ~-0.2%) is ~1.4% of notional, vs the OKX 0.15% floor. Risk route (b): signal/time exit with capped sizing, 15% of equity per alt leg (45% long gross), BTC short ~48%, gross <= ~1.0x, leverage 1x, no stop invented (the fixed clock is the exit); worst measured event ~-8% of equity.