[PK]

As I understand it (and I am not an expert here!), adding an air-ram mode to an engine means that it heats up and expels air instead of using any of its usual fuel. It takes the same built-in power source which would normally be used to drive its reaction engine and diverts that power over to an element which essentially overheats intaken air.

So, to reference your last example, no antimatter is involved. It doesn't slam antimatter into normal air -- it uses the energy which would normally contain and redirect the M/AM interaction to instead "ram" the air.

At least, that's how I understood it. I tend to just wave my hands at vehicles in a game. :)