I don't think the big move (up or down) happens until after Powell speaks tomorrow.
Rates and QE do impact the market, Powell managed to tank the market a couple of years ago by raising rates. He won't do anything tomorrow, but watch the market whipsaw like crazy anyways. That's what always happens.
SPX and NDX have overlapping stocks like AAPL, AMZN, GOOGL. That why they tend to move together but if you look at teh chart for last year you can see a lot of days where tech (NDX) was green but SPX went red. Learn how to read a chart please.