Yield Curve
Index VWAP
Index Vol
Constituent Vol
Index Differentials (spread market)
But really, it's about liquidity. If a large move has occurred, or is setting up, that's where you will see many traps, backfill, reversals, spikes, counter trend moves, etc. You need to know who is trapped, who is being forced to pay-up on losers, and who is in control. The index trades around the liquidity, which is based on the open and close of each exchange, and when the cash trading sessions overlap. The CBOT and cash treasury market (and foreign) trades in parallel.
The Asian session is a thing, and the EU and UK session overlap into the NYSE open is peak liquidity.
A lot is happening off-exchange (OTC), in dark pools, and in the basis trades (index basis). For example, when the Nikkei is trading on TSE, OSE and SGX during the ASX, HKEX and Shanghai sessions, that represents a lot of liquidity.