Off the top of my head: They are adding Cesium ("screw the environment, I have to escape my pursuer") to the engine in order to leave a plasmonic plume. The plasma, which occurs because the electrons in Cesium are stripped off in the heat of the engine, absorbs microwaves (ie, radar) because of the expected cyclotron physics of a free-electron gas.
However as an electronic countermeasure, I do not see the point. The plasmonic plume will absorb the microwaves where the rest of the plane will continue to reflect it. To my thinking, the is exactly what you do not want to do: You've reduced the microwave signal where the plane is not present, and done nothing to the signal where the plane is present. So you've improved the signal-to-noise for the enemy, which are providing guidance for anti-aircraft missiles.
Considering how many years this program went on for, my thinking must be wrong. Maybe anti-aircraft missiles of the era did not intercept from oblique angles (which takes serious computation and sensors) but could only hit from behind (where the plume would attenuate a signal)?