An air induction system brought in by Toyota for their 4A-GE engine.
Just before each pair of inlet valves, there is a pair of small butterfly valves, which operate like miniature throttle bodies. At lower revs (in the case of the 4A-GE, up until 3,500rpm) one of these valves remains closed, beginning to come open at 3,500rpm, and being fully open at 5,000rpm. This is accompanied by a 'whistling' noise which many mistake for the sound of a turbocharger coming on boost.
The idea behind this system is that the increased gas velocity (as a result of the smaller opening) gives the engine more torque in low rev ranges, making it more driveable, and also more economical.
Not to be confused with VVTi or VTEC, which are variable valve timing systems by Toyota and Honda (respectively).