I wouldn’t say it’s a new thing — lots of papers studied the impact of liquidity and flow in stock prices dating back to the 70s. What is interesting is approximating the multiplier effect.
TrimTabs International Free Cash Flow ETF (TTAI) Outgained $SPX +16.26% since March 2020 swing lows. Fund only came into existence 4 months prior so no other practical comparison can be made. Although the founder Charles Biderman has been an expert on this subject since at least the mid 90's that I know of. Curiously also the fund made a low March 12, '20 or 11 days prior to $SPX. Canary in a coalmine for the next drop? edit: "Fund only came into existence 4 months prior" - actually looked at chart again and it has been open since mid 2017 and sideways mostly, till March '20 drop and then pop. Return since as I said above.