When the distortion field is off - both a and b press into the vacuum "box" or keeping to our example the tailgate.
When side b is activated the - the distortion field blocks side b from having a pressure reaction into the core or tailgate in this case.
When a is activated, and b is inert - the pressure wave comes from side b.
When both sides are activated - the result is the same as if they were both off. With distortion fields into both sides, there are no pressure waves reacting to the vacuum core.
https://oi1341.photobucket.com/albums/o759/clackamascoug/2%20Guys%20pushing%20car%2028_zpst7ri9pp0.png