ETF shares are created and destroyed all the, bringing their value inline with the underlying. So for a S&P 500 ETF, HFT firms are creating shares when the pric

ETF shares are created and destroyed all the, bringing their value inline with the underlying. So for a S&P 500 ETF, HFT firms are creating shares when the price of the ETF is too low and destroying them when they are too high. It doesn't have to do with how ETFs rebalance, rather how shares are created and destroyed.