This is an aspect of quantum mechanics which applies to the fact that any oscillating frequency (such as a brainwave) reaches two points of complete rest which constitute the boundaries of each individual oscillation (i.e. movement up or down).
To enter these intervening dimensions, human consciousness must focus with such intense coherence that the frequency of the energy pattern which comprises that consciousness (i.e. the brainwave output) can accelerate to the point where the resulting frequency pattern, if displayed on an oscilloscope, would look virtually like a solid line.