Most banks have a ‘sweep facility’. When it is enabled and your balance crosses a threshold, it’ll put the money into something like an FD. The interest rates will be lower than actual FD but higher than savings account. No lock-in period or penalty either.
Assume threshold of 10k and after depositing some money, your balance went up to 12k. The bank will create a sweep deposit of 10k and the main balance will show as 2k. However, total account balance is still 12k. If you go to an ATM or a branch, you can still withdraw 12k. When you try to withdraw, the bank will look at the main balance first and if that’s not sufficient, it’ll apply interest to how many ever days has passed since the sweep deposit was created and then break off that sweep deposit.
Also, if your bank requires to have a minimum balance of 5k, the sweep balance will count towards it. So, if the main balance is 2k and sweep deposit is 10k, your total balance is 12k and so won’t incur any minimum balance penalty.