Both QQQ and XBI made a bullish reversal open on 27 Sep., however QQQ sold down while XBI moved higher.
When the daily candle on XBI (or candle pattern) does not match that on QQQ, take note. I have added lines to highlight examples, and these mismatches often occur at turning points with XBI indicating direction for QQQ.
It is not a sure predictor, as you can find counter examples also. See end of Jul. and 15 Sep. In the past few months I have found it useful to look at XBI and Bitcoin daily charts when evaluating direction for SPX and NDX.