The two forces or states are polar meaning they are polar opposites to each other and each have two poles or bipolar. The expanding force is the magnetic force having two equal poles mutually repelling each other. This force corresponds to the dissipative seventh (enharmonic) in Keely's music jargon. The contracting force has two poles and is the electric force having two poles mutually seeking each other through voiding. This force corresponds to the fundamental or unison or OM (Harmony).
Figure 3.36 - Contracting and Expanding Duality (click to enlarge)