Credit card checkers come with a program which performs a complex mathematical algorithm to solve this problem. Windows Checker comes with its own patented algorithm that is based on patented algorithms developed by award winning computer scientists. The patented algorithm takes the customer's last four digits of any debit or credit card number and creates a unique checksum.
That is all there is to Windows Checker. It will perform the basic operation of a normal credit card checker, such as storing customer data in its secure server, generating and comparing unique checksums to verify the authenticity of a transaction, and reporting any discrepancies it finds. But the unique thing about Windows Checker is that it can also perform a more advanced operation called "linking". If a customer pays with a debit card and the check amount falls under a certain range, then Windows Checker will compare the range with a pre-determined range of other ranges using the luhn algorithm.