1. It's an overdraft protection LOC for your checking account. Any check or debit that comes through that would have averdrawn your checking account will be covered by a transfer from your LOC to your checking account, up to the full amount of your LOC.
2. You can also draw on it as a LOC directly by asking for an order of NavCheck checks. This allows you to draw on your LOC without emptying your checking account first.
Tip: If you're out of NavCheck checks and need to access your LOC directly without emptying all the cash from your checking account, simply write a check from your checking account and trasfer most/all of the money from your checking to your savings until the check clears (so it draws on your LOC). Once the check clears, move your cash back to checking.
Life is a gamble. Get out there, roll the dice and see what happens. You can't win if you don't play!