Piracetam is not a potentiator of any other drug, at least not in the classic sense of the word. Rather, it increases certain types of brain functioning, and other nonspecific types like mitochondrial metabolic rate, oxygen transport, etc.
These improvements naturally mean improved response to the secondary substance(s). For depressants, this means lowered response because the brain is more active and capable to maintain its activity in the face of a push toward decreased activity.
For stimulants and psychedelics, it means an increase in perceived effect because, to put it simply, the brain is working better. It is lubricated. Thus the losses due to friction in this marvelous machine are reduced by a large amount, allowing it to attain high velocities with minimal wear naturally! That is the secret of piracetam: its lubricant function.
With chemofriction and metabolic traction losses decreased by a substantial amount, mentational processes can reach very high velocities. They are not forced to do so by better lubrication, but rather they are enabled to do so. Regular reports of even a mild stimulant such as caffeine causing jitters and excess stimulation in habituated users when combined with piracetam corroborate this proposition. Some brains which have a tendency to go hyper spontaneously may do so with piracetam alone but they are a minority. These brains usually belong to individuals who are close to the 'edge' of sanity, often require depressant psychomedication to stay 'on the level', or have bipolar, schizoid, or other disorders.
A clasic potentiatiation scenario is alcohol with benzodiazepines, or a MAO-metabolized psychedelic with a MAO inhibitor. In both cases the actual effect may be greater than the sum of individual effects. At minimum, the effect is additive, at maximum, multiplicative.