Here is my understanding. Let a<b<c be small numbers. Some investor thinks a stock is worth
x+c. They put in a limit buy order at x. Some HFT firm sees this and thinks well if they want it at x, I want it at x, and puts in their own order at x (+a fees to robin hood). Millenial comes and wants to sell their stock.
Normally the investor would get the stock since they placed their order first. But since the HFT firm is paying for the order they get it instead. If things go well the HFT firm can sell to the investor at x+b, if things go poorly they cut their losses and sell at x.
The investor that didn't get the order and has to buy it from the HFT firm at x+b is the loser.
The money that funds this dance comes from the millennial who sold a stock worth x+c at x, but that would have happened regardless.
I can't speak to the accuracy of your claim, but Matt Levine recently wrote about another factor, payment for order flow [0]:
>They want to buy stock for $99.99 and sell it at $100.01 and clip two cents on each trade. If their orders are random—if sometimes people buy and sometimes they sell, with no pattern—then that works out well for the market makers. But their big risk is what they call “adverse selection”: Sometimes, when a customer buys 100 shares at $100.01, it then buys another 100 shares at $100.02, and another 100 shares at $100.03, and keeps going until it has bought 10,000 shares and pushed the price up dramatically. The market maker who sold it the first 100 shares—and who is probably now short and needs to go out and buy those shares at a higher price—has been run over.
>[...] [I]f a market maker can guarantee that it will only interact with retail customers—if it can filter out big orders from institutional investors—then its risk of adverse selection goes way down. The way the market maker does this is by paying retail brokers to send it their order flow, and promising those brokers that it will execute their orders better* than the public markets would. [...] It can offer a tighter spread than the public markets—and have money left over to pay the retail brokers—because it doesn’t have to worry about adverse selection. If the retail broker is, say, one designed to let young people day-trade for free on their phones, then those orders are probably particularly valuable, because they are probably particularly random.*
Normally the investor would get the stock since they placed their order first. But since the HFT firm is paying for the order they get it instead. If things go well the HFT firm can sell to the investor at x+b, if things go poorly they cut their losses and sell at x.
The investor that didn't get the order and has to buy it from the HFT firm at x+b is the loser.
The money that funds this dance comes from the millennial who sold a stock worth x+c at x, but that would have happened regardless.