A single-instrument volatility-timing strategy on QQQ.USEQ daily bars. Every 21 trading sessions, the strategy closes its QQQ position and reopens it at a new notional: target_vol / realized_vol_21d of equity, capped at 1.8x on the 2x Reg-T margin account. Realized vol is the annualized standard deviation of the trailing N daily close-to-close returns (sqrt(252)). The mechanism comes from Moreira & Muir (2017), 'Volatility-Managed Portfolios', Journal of Finance. Equity-index volatility clusters strongly, but expected returns do not rise in proportion to it. Cutting exposure when recent vol is high and adding it when vol is low therefore raises the return per unit of risk. The economic cause is the leverage effect plus volatility persistence. It is not price momentum: no trend, breakout or signal on price direction is used. The 21-session re-strike is a fixed bar count from the first tradable bar, NOT a calendar date (lesson L179), and it exists only so each holding period is a separately measurable trade. POINT-IN-TIME PRE-STUDY on the local catalog (QQQ.USEQ 1-DAY, 1999-2026, net of 3 bp of turnover cost): buy-and-hold had Sharpe 0.50 and peak-to-trough DD ~83% (2000-02). VM21 with a 15% target had Sharpe 0.72 full-sample (0.50 pre-2016, 1.04 post-2016) and max DD ~22%. Lookbacks of 10, 21 and 42 sessions all gave 0.68-0.72, so there is no parameter cliff. On SPY the same rule raised Sharpe from 0.55 to 0.61, which confirms direction but is weaker, so QQQ (higher vol clustering) is the primary. NEGATIVE CONTROLS from the same pre-study, which I am NOT proposing: dollar-neutral sector-ETF 3-12 month momentum (9 SPDR sectors) gave Sharpe -0.25 to +0.05 everywhere. Adding a 150/200-SMA trend gate, a TLT defensive sleeve or a short leg to VM21 did not improve it (0.52-0.67). The edge is the vol scaling alone, so only that is proposed. Venue rationale: USEQ is 4.8% of the corpus; QQQ has decades of daily history; execution is commission-free. It is long-only (an over-represented bucket) because the pre-study showed the short-side variants lose Sharpe.