BrainWaves

delta (0.5-3.5 cycles per second), 
generally found in sleeping subjects.

theta (4-7 cycles per second), 
a hypnogogic state

alpha (8-13 cycles per second), 
associated with a relaxed, alert state

beta (14-30 cycles per second), 
produced during states of agitation and intense mental concentration. 

The hemi-synch - and related mind-machines - 
can produce alpha or theta waves, on demand, 
according to the operator's wishes. 

A suitably-entrained brain is much more responsive to suggestion, 
and is even likely to experience vivid hallucinations.

This rhythm was first discovered in 1924 by the German psychiatrist 
Hans Berger, who recorded cerebral voltages as part of a telepathy study.