Ok - here's my best explanation of why @RobinhoodApp restricted trading in the short-squeeze stocks.

Spoiler: the story isn't the Ken Griffen called Janet Yellen who instructed DTCC to raise margin on Robinhood to force them to shut down the speculative buying.

Here goes ...

Robinhood (RH) is a broker. They don't execute stock orders themselves. They sign up customers, route their orders to executing brokers, and keep track of who owns what. RH is also its own clearing broker, so they directly settle and custody their clients' securities.

Yes, RH is paid by Citadel to handle executing some of its order flow. This isn't as nefarious as it sounds - Citadel Equity Securities is paying to execute retail orders because they aren't pernicious (like having 500x the size behind them).

RH customers buy and sell stocks. Those trades don't settle (settle = closing, the exchange of cash for security) until T+2, two days later. Depending on the net of buys/sells, RH is on the hook to pay or recieve that net cash. That's credit risk.

NSCC is the entity that takes that credit risk. It matches up the net buyers and sellers, post-trade, and handles the exchange of cash for security. To mitigate the credit risk that one of the clearing brokers fails, they demand the brokers post a clearing deposit with them.