miller cycle (as opposed to the usual otto cycle gasoline engine) is a modification where the intake valves are held open during the compression stroke. this reduces pumping losses and increases engine efficiency, at the expense of reduced displacement (due to some intake air getting pushed out again).

the KJ-ZEM is based on a 3.0L V6 but only displaces 2.3L due to the loss of displacement. however it produces the power of the 3.0L V6 with the fuel consumption of a 2.3L.

the cycle is called "Miller cycle" if there is a roots-type supercharger on the intake to make up for the lost air. if there is no supercharger it's called an Atkinson-cycle. the Prius has an Atkinson-cycle gasoline engine.

the main use is really to make the system more fuel-efficient.