The trans operates via two clutches and two bands and an overrunning clutch. In D or L1, the rear clutch is applied. In L1, it is helped by the rear band. When the trans shifts to D(2nd) or L2, the front band is applied, holding the front drum. When shifting to D(3rd), the front clutch is applied, causing the front drum to spin. Simaltaneously, the band is forced to release. This means that pressure is holding the band applied, but a larger pressure is holding it released. The net effect is a faster "kickdown", producing the "snap" that you need when passing.
In reverse, the rear band and front clutch are applied, causing the output shaft to counter rotate.
All of this is accomplished using planetary gear sets. This allows the trans to have the gear ratios meshed at all times, so no "missed" shifts.