Each month they'll give a discount on the payment plan until the device is paid off, so $0 every month, but if you cancel early you'll owe the remainder of the payment plan.

For example you to get a $1200 phone for "free", each month on your bill would be $50 device payment plan and $50 credit to offset the payment plan. Do this for 24 months and the phone is paid off and it's yours.