The coils on each driver have a specific resistance, measured in ohms. When wired a certain way, that resistance across the coil is cut in half, wired another way it is doubled.
Example/
Parallel = /2
Series = *2
Two SVC 4ohm subs wired in parallel makes a final 2ohm resistance
Wired in series they produce 8ohms resistance
Two DVC 4ohm subs wired in parallel/parallel (the VC's on each sub wired in parallel and then the two subs wired together in parallel) produces a final load of 1ohm. 4/2=2, 2/2=1
Wired all in series would produce a 16ohm load. 4*2=8, 8*2=16
Help at all?