There are pressure pulsations in an intake manifold for obvious reasons. The amplitude and frequency of the pulsations depends on the intake manifold. As Lee said, one can tune the pulsation to increase VE at given conditions. One has to tune for steady state, alter valve timing or use a variable geometry intake manifold to achieve maxima VE in this method.
All the math you need is here,
http://www.amazon.com/Theory-Engine-...1397460&sr=8-2