Unstart is the supersonic choking reaction that occurs in ducts with an upstream mass flow greater than the downstream mass flow.
There are other ways of conceptualizing unstart which can be helpful. Unstart can be alternatively thought of in terms of a decreasing stagnation pressure inside of a supersonic duct; whereby the upstream stagnation pressure is greater than the downstream stagnation pressure. Unstart is also the result of a decreasing throat size in supersonic ducts. That is the entrance throat is larger than the diffusing throat. This change in throat size gives rise to the decreasing mass flow which defines unstart[1].
The choking reaction of unstart results in the formation of a shock wave inside of the duct.