This article contains information and a sample on manipulating Ektron Checkout Commerce control phases as described in the documentation.
The eCommerce server controls contain event hooks so you can customize or manipulate them. To use these event hooks, create a handler for each needed event in the code-behind of the ASPX page that contains an eCommerce server control. Then, in that page's OnInit method, hook each event.
PhaseChange - Used only in the Checkout server control and called when the current phase is about to change. Supervise phase changes and allow or prevent changing as needed. It also lets you jump to a specified phase. For example, you could jump to a passed phase, prevent the phase from changing from the current one under certain conditions, or jump to a phase that is not necessarily the next one in line.
For more information on the flow of the checkout server control please click here .