Each time VIX (Volatility Index for SPX) increases or rises significantly - SPX drops.
Watch VIX for breaks of trendlines or retests to predict SPX drops.
Whenever SPX trends down smoothly - SPX slowly climbs up. Then you may buy SPX.
If SPX breaks its downtrend (especially with a gap as we saw last year) or rebalances (restests) after the downtrend break (as we saw now in January) be prepared for sudden SPX drops.