Russell 1000 overlay??

In the CFAI EOC questions on Equity portfolio mgmt, Question 11 states that there is a long-short manager whose normal benchmark is ‘Cash with Russell 1000 overlay’. What does ‘Russell 1000 overlay’ mean?

Russell 1000 overlay is exposure to the Russell 1000 index using futures. Since the normal benchmark is Cash with Russell 1000 overlay, it means the manager has used the long-short strategy to create a market neutral exposure to equity and the equivalent amount of cash raised from shorting has been used to purchase t-bills which is kept as collateral against the Russell 1000 futures purchase.

interesting… thanks