It is called spoofing, and is illegal.
It's not spoofing Overnight. I'm talking about actual trading in the futures. These are completed trades here man. Actually, significant volumes of trading.
Spoofing is more of a hindrance for algorithmic traders that are using order flow. It's not even related.
I am referring to movement in the basis spread. The index basis 'floats' depending on demand for the hedging instrument, as well as fluctuations in the listed bids and offers on the underlying stocks. This is because futures and stocks trade within a no arbitrage spread most of the time.
These are charts of basis spreads. The upper chart is detrended, the lower is not.
For reference
https://www.cmegroup.com/education/...ndex-products/what-is-equity-index-basis.html