Large commodity index products track rules-based benchmarks that must stay in liquid contracts. Their rules move one-fifth of the position each day on business days five through nine of the month, selling the expiring contract and buying the next. Because the schedule is public, everyone knows the flow is coming.
Front-running the roll used to be a reliable trade: sell the front and buy the deferred ahead of the index, then hand the position back to them. Publication of the schedule and competition among traders has largely eroded the edge, but the flow still distorts intramarket-spread pricing in that window.
Example: an index holding 100,000 crude contracts rolls 20,000 per day for five days. Against average front-month daily volume of roughly a million contracts, that is a visible but not overwhelming push on the front-to-second spread.
Related: roll, roll-date, intramarket-spread, roll-yield, volume-vs-open-interest