Classical Conditioning normally affects unconcious processes, such as blinking or other various reflexes. It occurs by presenting a stimulus (one that invokes a reflex action), paired with another "neutral" stimulus (one that does not naturally invoke a reflex action) continuously to a subject until the body adapts/reacts to the stimulus. When the reflex-invoking stimulus is withdrawn, the previously neutral stimulus is sufficient to produce the reflex action.
Operant conditioning is the association of a behavior with the consequences it produces. Positive consequences increase the frequency a behavior is performed, while negative consequences decrease them.
Both are valuable tools for use in psi training. These techniques can make psi practice more natural and a part of daily living, as opposed to continuous mental effort. For clarification, operant conditioning could be used in the training of psi sphere generation, as the positive consequence would be the perception of stronger psi spheres. Other consequences are indeed possible, this one was used just for example. Classical conditioning is of course ideal for a reflex psi action, as one could train themself to generate shields every time a clock is seen, perhaps. The possibilities are endless, and all can serve to make psi a little less grueling for the serious practitioner.