  The very existence of the physical vacuum provides a framework to propose a
general mechanism for propelling bodies through an agency of electromagnetic
fields, that seat in that medium. When two sub-systems of a general closed
device interact via nonlocal and retarded electromagnetic pulses, it is easily
shown that they give a nonzero force, and that only tend to comply with the
action-to-reaction force in the limit of instantaneous interactions. The
arrangement of sub-systems provide a handy way to optimize the unbalanced EM
force with the concept of impedance matching. The general properties of the
differential electromagnetic force (DEF) are the following: i) it is
proportional to the square of the intensity and to the angular wave frequency
$\omega$; ii) to the space between the sub-systems (although in a non-linear
manner); iii) it is inversely proportional to the speed of interaction; iv)
when the two sub-systems are out-of-phase, DEF is null. The approach is of
interest to practical engineering principles of propulsion since it offers
guiding principles useful to build prototypes.
