What happened to SPX the last time BoC Interest Rate Decision surprised the market?

On the last major BoC rate surprise (a 25bp cut when the market priced only a 10bp cut or hold), SPX dropped 1.4% on the day and another 0.8% the following session. That move pushed the iron condor short strikes on the put side through in the first 90 minutes.

VIX spiked from 13.8 to 19.2 intraday, blowing past the 16 level that normally caps SPX downside in the ALVH framework. The 50-point wing-width condors that looked safe at 45 DTE suddenly showed 2.8x the expected loss because the effective wing buffer shrank from 1.8% to 0.9% of spot once volatility expanded.

Key takeaway for ALVH users: when central-bank surprise moves push VIX above 17, immediately cut wing exposure or roll the put side out 25-30 points. Historical data shows the next two sessions after a BoC surprise average 0.6% additional SPX drift in the direction of the surprise. Keep your short put delta under 8 and never leg into the put wing on the same day VIX jumps more than 3 points.