A sandbox example

Recent Trouble Among Money-Market Mutual Funds, and the Way Forward

There was a singular MMMF run in 2008. To summarize what I wrote about it here, the Reserve Primary Fund suffered losses when the failure of Lehman Brothers drove down the market value of its portfolio. The Fund neither injected capital nor promptly marked down its share price from its conventional $1 price (it did not "break the buck" until two days later). Shareholders saw that redeeming immediately would mean getting $1 per share, but those at the end of the line would get less. So they ran. The Reserve Primary Fund belatedly reduced its redemption price to 97 cents, meaning that the remaining customers took a 3 percent haircut.

Larry White talking about the Fed intermediating in the money market funds. 

If the fund managers immediately  mark the value of the fund according to the withdrawals (or deposits), then they remove arbitrage, no congestion. His analysis is all about congestion management, he is a sandboxer.