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Simply throttling at the valve through low lift or low duration doesn't rid you of pumping work losses since the piston (the pump) still sees vacuum during the intake stroke.
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You get some of that energy in pulling against the vacuum back during the compression stroke when the vacuum pulls the piston up. Throttle plates cause significant internal energy loss due to the turbulence they produce. It's not isentropic. Changing volumes by moving a piston can theoretically be isentropic. It's not because of the friction involved, but the friction is there anyway. So the wider you open the valve, the more efficient it will be since limiting the intake valve motion simply turns it into a throttle.