Single traded leg: SPY.USEQ on daily bars. TLT.USEQ is a SIGNAL-ONLY extra instrument and is never traded. Economic cause: balanced mandates (pensions, target-date and 60/40 funds) rebalance toward fixed equity/bond weights at month-end. When equities have outperformed bonds month-to-date they must SELL equities into the close of the month, and when equities have lagged they must BUY. This is a known, mechanical, price-insensitive flow, and it is documented in Harvey, Mazzoleni & Melone (2025, 'The Unintended Consequences of Rebalancing', NBER w33554): SPY returns over the last days of the month are negative after equities outperform bonds, and they partially reverse at the start of the next month once the flow is gone. The payer is the rebalancing fund, which trades at a known time regardless of price. Point-in-time rule, no fitted threshold: at the close of the SECOND-TO-LAST NYSE session of month m (computed from the XNYS calendar), compute G = SPY_close/SPY_close(last session of m-1) - TLT_close/TLT_close(last session of m-1). Leg A (month-end pressure): position = -sign(G) SPY at that close, held to the close of the LAST session of month m. Leg B (post-flow reversal): at the last close of month m, flip to +sign(G) and hold for 2 sessions, to the close of the 2nd session of month m+1. Then flat until the next month's signal. Pre-study, run by me point-in-time on catalog SPY/TLT 1-DAY bars from 2002-08 to 2026-08 (289 month-ends), 3bp per unit of turnover. Leg A: mean +0.236% per trade, t = 4.19. Leg B: mean +0.216% per trade, t = 2.35. Combined book at 1x: calendar Sharpe 0.58, 3.9%/yr, max DD 18.4%, 72% of years positive, in the market 14% of sessions. The first 80% of the sample (IS) has Sharpe 0.49 and the last 20% (2021-11 to 2026-08) has Sharpe 1.02, so the effect has not decayed. Attribution control: the same rule with the bond leg removed (SPY month-to-date return only, i.e. plain short-term reversal) drops to Sharpe 0.24, with Leg A t = 0.74 and last-20% Sharpe -0.67. The information is therefore in the equity-vs-bond gap, not in SPY's own reversal. Robustness: the result holds with IEF or AGG as the bond leg (Leg A t = 3.0 and 3.2), and with signal day T-3 instead of T-2 (t = 3.08). Distinct from the abandoned IEF month-end duration-extension trade, which was an unconditional long-only calendar hold with no cross-asset signal, and from the abandoned SPY weekly majority/minority selector, which is a pure own-price scheme. Mechanism stripped of instrument and timeframe: 'trade against the forced month-end flow implied by a balanced fund's equity-vs-bond drift, then against its unwind'. That sentence has not been run in the corpus. Deliberately ignored library excerpts: none of the shown library documents apply.