From matt levine:
>This means that the seller takes two days of credit risk to the buyer. I see a stock trading at $400 on Monday, I push the button to buy it, I buy it from you at $400. On Tuesday the stock drops to $20. On Wednesday you show up with the stock that I bought on Monday, and you ask me for my $400. I am no longer super jazzed to give it to you. I might find a reason not to pay you. The reason might be that I’m bankrupt, from buying all that stock for $400 on Monday.
>Generally if you buy a stock on Monday you still want it on Wednesday; even if you don’t, we live in a society, and you’ll probably cough up the money anyway because that’s what you’re supposed to do. But at some level of volatility things break down. If a stock is really worth $400 on Monday and $20 on Wednesday, there is a risk that a lot of the people who bought it on Monday won’t show up with cash on Wednesday. Something very bad happened to them between Monday and Wednesday; some of them might not have made it. You need to make sure the collateral is sufficient to cover that risk. The more likely it is that a stock will go from $400 to $20, or $20 to $400 for that matter, the more collateral you need.
https://www.bloomberg.com/opinion/articles/2021-01-29/reddit...