How the machines work vary depending on the machine and POS setup. There are some that scan the check and ping the associated bank account to perform payment routing in an eCheck format, but the vast majority that I worked with just franked the check (which if you didn’t know is a glorified stamp with the transaction and deposit information for the bank to indicate the check was a part of a retail transaction). With the lower end printer models, you could run a blank piece of paper through it and it wouldn’t know the difference.