From February 5th:

Okay let’s do payment for order flow again, because people are talking about it and that always stresses me out. Here’s an intuitive description of how it works. A million people come to a broker to trade GameStop Corp. stock. Half of them want to buy shares, half of them want to sell shares. One share each, all using market orders, all at precisely the same time. The stock exchange has half a million shares of GameStop available for sale at $58.25, and orders to buy half a million shares for $58. The broker could send all of its customers’ orders to the stock exchange, where the buy orders would be filled at $58.25 and the sell orders would be filled at $58; the broker would pay the exchange a small fee for executing these orders.

But! The broker realizes, look, all these people who want to buy shares could be matched up with all these people who want to sell shares. I don’t have to pay a fee to the exchange, the buyers don’t have to pay $58.25, and the sellers don’t have to get $58. The buyers could pay $58.15 and save 10 cents, the sellers could get $58.10 and make an extra 10 cents, and I could keep 5 cents (and avoid the exchange’s fee) for my trouble. That’s a good deal for everyone!

This is called “internalizing”: Your broker executes your order internally, against its other orders, rather than sending it out to the exchange. In practice a typical retail broker doesn’t have the ability to do this, so it sends its orders to what is usually, in the business, called a “wholesaler” (or sometimes “internalizer”), and usually, outside of the business, called a “high-frequency trader.”[3] Popular wholesalers include Citadel Securities, G1X Execution Services LLC, Two Sigma Securities LLC, Virtu Financial Inc., Wolverine Securities, etc. The wholesaler does the thing I just said: It pays the sellers more for their shares than the exchange offers, charges the buyers less for shares than the exchange would, and keeps 5 cents for itself. Well, it keeps, say, 3 cents for itself, and sends 2 cents back to the retail broker who sent it the trade. The broker has subcontracted the internalizing job to the wholesaler, and they share the profits.

Also a good debunking of this myth:

Otherwise normal people will start out mainstream explainer articles by saying, like, “Robinhood sells your order to Citadel so Citadel can front-run it.” No! First of all, it is illegal to front-run your order, and the Securities and Exchange Commission does, you know, keep an eye on this stuff. Second, the wholesaler is ordinarily filling your order at a price that is better than what’s available in the public market, so “front-running”—going out and buying on the stock exchange and then turning around and selling to you at a profit—doesn’t work. Third, because retail orders are generally uninformative, the wholesaler is not rubbing its hands together being like “bwahahaha now I know that Matt Levine is buying GameStop, it will definitely go up, I must buy a ton of it before he gets any!” The whole story is widely accepted but also completely transparent nonsense.

Posted here for all eternity, as this is a good reference and quick explanation about what PFOF is. Handy to have around.

Find Matt Levine at Bloomberg.