When you enter phase state, you will phase out when your combat spell fails and enemy tries to hit you in revenge. It is very useful at creatures who deal really great hits and are able to kill you by one blow. But in this phase state, all lost concentrations cost twice as much as in normal state. In phse state you also sometimes phase out when your spell is reflected back to you, but to deal with them it is better to rely on spell guidance.

See also: spell guidance