When you select a service plan with Overage Protection Off, you automatically roll up to the next highest plan after you consume your starting plan.
For example, if you are on the 1 GB data plan with Overage Protection Off, when you use the 1 GB of data, you are moved to the 2 GB data plan automatically. However, you are only charged the difference of the two plans as you receive the difference in the amount of data included in the two plans. So, in this case, you would receive an additional 1 GB of data and be charged $4.00 (not including taxes).
With Twigby, there are never any additional fees or penalties for going over your plan. You simply pay the difference to go to the next highest plan. Of course, if you decide you never want to be charged more during a month, you can simply turn Overage Protection On to be protected from overage charges.