Suppose you deposit $1,000.00 into your bank. Before loaned by the bank, the money belongs to you (unless you consider your act of depositing it as a donation to the bank). When the bank loans it, the money goes into the borrower's account without living yours. Then, both of you can spend it, regardless of whether at the same time or not, and whether by withdrawing it from the bank or not, which is precisely why the money supply increases (to simplify things, imagine the borrower has an account in the same bank as you and after the loan is credited in his account both of you simultaneously spend that money merely by transferring it to other two accounts in the same bank).
The moment you deposit your $1000 into your bank, they are not yours anymore, you are only left with a certificate that they belong to you. (your account statements). What really happened is that you loaned these money to the bank, and now bank take over the ownership, leave you with an IOU statement. If the bank shut down the second day, you lose all your money, but that risk fall upon you as soon as you put your money in the bank, not when bank shutdown (comparing this with depositing your money to MTGOX
)
And then banks loaned those money to another people. When you try to spend your money, they will provide you with corresponding base money to spend
Since they always have a base money buffer with millions of dollars to deal with the daily transaction, so you might have an impression that both of you can spend the same money at the same time (In fact both of you are just spending a small part of a large base money pool that grows and shrinks with each custom withdraw/deposit). Since banks usually put a daily withdraw limit on each account, this pool will handle the daily variation very well. Normally 500 customers will make this pool very stable
But if all those 500 customers spend at the same time, things will be different. In financial crisis, people were panic selling assets and billions of money are requested to be withdrew from a certain bank, and they just don't have enough base money to deal with it
So, FRB is like insurance, as long as majority of people feel confident with their money in bank, there is no problem, but if there is a panic and all the people start to withdraw at the same time, they will find out there is only 10% of their account deposit in the bank that can be withdrew
Of course banks are pushing all-electronic banking so that all the money never left this large base money pool. I'm still studying the possible consequence of this move