Can anyone explain the $400 rule in Reg. CC or give me a website to go to that explains this? I know about the $100 rule but I need info on the $400 rule.

From Regulation CC

Sec. 229.12 Availability schedule
(d) Time period adjustment for withdrawal by cash or similar means. A depositary bank may extend by one business day the time that funds deposited in an account by one or more checks subject to paragraphs (b), (c), or (f) of this section are available for withdrawal by cash or similar means. Similar means include electronic payment, issuance of a cashier's or teller's check, or certification of a check, or other irrevocable commitment to pay, but do not include the granting of credit to a bank, a Federal Reserve Bank, or a Federal Home Loan Bank that presents a check to the depositary bank for payment. A depositary bank shall, however, make $400 of these funds available for withdrawal by cash or similar means not later than 5:00 p.m. on the business day on which the funds are available under paragraphs (b), (c), or (f) of this section. This $400 is in addition to the $100 available under Sec. 229.10(c)(1)(vii).

The $400 rule requires that you have a processing system capable of incrementing up availability at 5 p.m.

Let's say you have a non-local check of $1,000 deposited on Monday. You have to make $100 available for withdrawal on Tuesday, and the balance of the check should be made available for withdrawal on Monday of the following week (five business days). If you use the $400 rule, you can prevent your depositor from withdrawing CASH from that $900 on the fifth day (Monday) until 5 p.m. At 5 p.m., you have to free up cash access of $400, and the remaining $500 must be available for cash withdrawal by the opening of business (or 9 a.m. at latest) on Tuesday.

That only applies to cash access. So it could prevent ATM withdrawals, check certification, teller withdrawals, POS authorizations. At 4 p.m., the balances against which ATM and POS transactions are authorized would have to be refreshed with the newly-available $400. And all day long on Monday, any inclearing checks would have to be paid against that $900 (remember, it only affects cash access).

The wrinkle was a "bone" that got thrown to bankers who argued that cash access on day 2 or 5 would expose them to too much check fraud risk.