See also: "Can Vmg exceed the V of a jet exhaust?"
There was a similar discussion last year, Can Vmg exceed the V of a jet exhaust?, in which ChristiaanJ (I think) reasonably criticized my dislike of the `free lunch'-type of description of supersonic intakes, owing to having been there and knowing the load-bearing structure of the concorde intakes.
The pressure of the air increases going backwards through the intake, and this produces a net forward force on the intake ramp. As M2dude's quote rightly states, the engine is enabling this to happen. Turn off the fuel and the thrust from all bits of the intake-engine-nozzle system no longer occurs.