Stands for Sequential Valve Timing - Mazda's answer to VTEC (Honda)and VVTi (Toyota).

A S-VT engine continuously varies the opening and closing timing on the intake valves and consequently optimises valve timing under all conditions (which VTEC doesn't if I recall).

From the Mazda web site as I don't really understand this technology fully

In order to transfer the rotation of the crankshaft to the camshaft, a pulley is attached to the front end of each shaft. These are linked by a timing belt. In most engines, the pulley is attached directly to the camshaft, which determines the valve timing.

In the S-VT engine, however, a vane actuator attached to the pulley part of the intake camshaft is used to facilitate continuous adjustment of the valve timing. The vane actuator comprises a housing fixed to the pulley and a vane fixed to the camshaft, with the hydraulic chambers of the advancing chamber and delaying chamber systems between the housing and the vane. The hydraulic pressure applied to these hydraulic chambers is controlled by an oil control valve (OCV), whereby the relative phases of the pulley and the camshaft can be varied and the intake valve timingcontrolled. The OCV also detects engine rpm, the amount of air taken in, and water temperature by means of a computer so that it can perform optimal control of valve timing under all driving conditions.