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There's 2 parts to it. The Valvematic (and VVEL/Valvetronic) essentially adds an adjustable rocker between the camshaft and the valve actuator so you can change essentially the point at which the cam is "engaged". This means lift and duration are decreased together, which is why the peak torque on the Valvematic engines can be a little bit lower (though the powerband is likely going to be better overall).
If you were to have say an adjustable rocker arm that only changed lift but not duration, that would give you independent throttle bodies right at the valve. This is better than a throttle body because once the valve is closed no air leaks through, so you don't have to draw as deep of a vacuum as say an independent throttle body setup because while the intake is closed the air is still leaking in. It's also better because 100% of the vacuum is contained in the cylinder, meaning your losses are exactly the increase in entropy from the expansion of the air into the cylinder, no more (draw some PV diagrams to see this). With a throttle, those losses look smaller at first, but then since air is constantly leaking through the throttle you end up with more losses.
Being able to adjust duration though means you get some air metering capability that does not cost you pumping power, which is where most of the gain comes from.
Another benefit is that with less lift you get more favorable flow velocity (aka better fuel vaporization) at low speeds.
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