The manufacturer can use a combination of a DC motor, speed controller, and the angle of
the swash plate to control hybrid assist and / or regenerative braking.
In fact, some compressors [very expensive German ones] use a variable
swash plate that can change the stroke (displacement) of the compressor on the fly.
With these compressors, checking for pressures is pretty nonsense, because they can adjust the pressures in the system by themselves according to the cooling demand, all they have to do is reduce
the swash plate / driver angle.