I was chatting with a guy on a trading message board about high frequency trading (HFT) explaining that HFT can move the market (e.g. the S&P 500) in an illiquid market, and he says "My point, which still stands, is that HFT has zero impact on moving the S&P 500 index. By definition if HFT was taking positions, it wouldn't be considered HFT. If by HFT you meant automated orders then of course if the algorithms kept hitting stops and continued buying it would impact prices. But algos and automated orders that take on positions have nothing to do with HFT." He seems to think that because HFT only holds trades for milliseconds that it's not considered "taking a position" and thus can't move the S&P 500 (or another large market index). Is he correct or not? If not, why?