builds torque because it scavenges the exhaust fumes and creates a "low" pressure, almost vacuum like.
When the right side fires, it creates a low pressure vacuum in the left pipe, so when the left side fires it essentially sucks the fumes out of the chamber as well as the engine pushes them out. So you flow more/better and build better power. plus an equalizer pipe or a cross-over will smooth out idle and I personally think they sound much better than straight duals.