What mechanism or design aspect prevents jet blast from escaping the front of a Ramjet or scram jet? Enormous air pressures result in the thrust of a Scram jet or ramjet engine, more so the pressure required to drive the aircraft forward at supersonic and hypersonic speeds.  Given that these engines are designed to function without a  fan which sucks air and at those speeds would prevent air from coming in too fast or going back where at came from post being heated.  What mechanism or design aspect prevents jet blast from escaping out the front of a Ramjet or scram jet, resulting in lost thrust and or perhaps negative thrust given that explosive pressure would drive air in any direction that would allow it to escape.  

I'm also considering that the air entering the engine does not have that much pressure to prevent this cause if jet blast did exit out at the front, scram jets and ram jets would not be able to produce thrust
Pressure is the greatest just before the combustor in both jet and ramjet engines - therefore air would have against the pressure differential for this to happen (turbine only has therefore an indirect role)