a DVC or dual voice coil sub has two coils, or two pairs of speaker terminals. you need to wire them together. if you wire them together in parallel (pos to pos, neg to neg) you get half the impedance of a single coil.. so a dual 4 ohm sub would be a 2 ohm sub when wired with coils in parallel together. if you wire the coils in series, teh impedance doubles, so a dual 4 ohm coiled sub would be 8 ohms when coils are wired together in series (pos to neg, neg to pos)
once the coils are wired together, you just wire one pair of speaker terminals to the amplifier's pos and neg terminals as normal.
there is no difference in sound. this is merely done to allow more wiring options to match the amp you choose to use to get ideal power from the amp.