Nissan's official statement on the subject says the Serena S-Hybrid is equipped with "a simple and compact hybrid system. It mounts an upgraded version of the ECO motor used in the existing Serena, and serves as an auxiliary motor to increase power output and power generation capabilities."
Automotive News says the the Serena's S-Hybrid regenerative braking system mostly converts kinetic energy into electricity to power the minivan's electrical systems, like the taillights, and thus lets the engine operate in a more efficient manner. The end result? Improved fuel economy of up to 10 percent, or around $1,900.