All things in the financial world related to Sep are expiring today, including the index futures contracts that are cash settled. If one were to have acquired a bit of length in the instrument, they could not only caryy it into close, but buy all they wanted to into said close thus propping. After close, the register is automatically "rung", and after clearing, you would have a very large pile of cash with no market position.
Also coincidental is that money market guarantees expire now, how fitting for the times.
This is the point I would not want to be holding length anymore.
