The documentation on "Become an anchor>Architecture" says the first option is "Maintain a Stellar account for each customer. When a customer deposits money with your institution, you should pay an equivalent amount of your custom asset into the customer’s Stellar account from your base account. When a customer needs to obtain physical currency from you, deduct the equivalent amount of your custom asset from their Stellar account."
I'm baffled as to how I can take money from a customer's account. Are the asset anchors able to arbitrarily DB any stellar account holding that asset? if not, what does the last sentence mean?