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In a resonant cavity or a gap, with a nanometer-sized width and depth on an Au surface, filled up with SiO2, the electric field intensity is enhanced by many orders of magnitude by the illumination of a plane wave. Graphical representations of power flow are employed to elucidate how the cavity harvests energy from the incident wave inside to give rise to the enormous field enhancement. The power income from the incident fields into the cavity and the power expense as scattered fields and absorption are discussed on the basis of three Poynting vectors, which correspond to the extinction, scattering, and absorption cross sections. The streamlines of the Poynting vectors distinctly visualize that the cavity collects light from an area much wider than its own geometrical width. In addition, the presence of an alternating power flow accumulated in the cavity for a certain time duration is unveiled on the basis of the real and the imaginary parts of the complex Poynting vector. Both the spatial squeezing and the temporal accumulation contribute to the high power density in the cavity. A clear insight into the underlying physics acquired on the basis of a pictorial understanding is expected to play a critical role in designing a plasmon nanocavity with a higher field enhancement in a rational way.