When you buy an item at a store with a check or credit card, you are promising that the money is available in your account. The store doesn't receive the money for up to 10 banking days later.
Sending a check to Escrow.com works exactly the same way. To ensure payment, Escrow.com waits until the money is deposited into our trust account, which is then considered "good funds." This takes longer, but gives Buyers and Sellers an extra level of security.