When you send funds to your account, it is possible to use them before the value date. Funds become available immediately for further operations as soon as they appear in your Transactions not booked under Account details - click the information i icon on the right side from account number.
The amount booked will move from Transactions not booked to Cash balance on the following day.
The use of cash before value date will have an impact on the interest calculation as interest is calculated on the value dated cash balance and not posting date (value date and posting date of a cash transaction are visible in your account statement).
In case you have negative free equity on the account, you will be charged negative interest.