There is an image that is firmly fixed in the imagination of the Western society. It envisions witches gathered in some lonely place or elsewhere in secret under the light of the full moon to engage in their craft. A related, much more modern addition to this folkloric image, is that of a cult of occultists gathered during the new moon engaged in some sort of 'dark' ritual. Usually both images are something that is supposed to be shocking and lurid. While these images are false, they do touch upon something that is true.
Through out a given month, there are times where the psychic energy of the world is more accessible than others. The energies of increase are tied to the waxing of the moon, reaching their apex with the full moon. The energies of decrease are tied to the waning of the moon, reaching their greatest strength with the night of the new moon. Spells that are aligned with concepts of increase are most effective when the tide of increasing energy is rising. The spells aligned with concepts of decrease are most effective when the tide of decreasing energy is rising.
It is possible to do a spell focused upon increasing or decreasing during the time where the psychic tides are focused elsewhere. The spell may need to be reworded and redesigned to accommodate how the world's energy flows. Astrological orientation has an influence upon how the world's energy flows as well, but it is complementary to the major psychic tides influenced by the moon.
The full moon is a unique position that is especially well suited for general purpose magic. It is the apex of the energies of increase but it is also the final stage of it, thus lending an emphasis upon the aspects of decrease. The new moon is similar but it has less of a tie to the energies of increase than the full moon does to the energies of decrease. The first few degrees of the moon illuminated following the new moon are more strongly tied to the energies of increase than the new moon itself. As such, many treat the new moon as the 'dark' moon and the time where the energies of decrease are greatest and the energies of increase are functionally not at play.
Some argue that the shift from increase to decrease is the ebbing of psychic tides and the shift from decrease to increase are the rising of the psychic tide. This is actually among the most common perspectives on this matter. Thus when a general practitioner of modern, Western pagan magic states that the psychic tide is high, they're referring to the energies of increase. I have moved away from this position because it does not accurately reflect that the opposing energies are present at all times.
The psychic energies of increase and decrease can not be completely divorced from each other. They are tied together in a symbiotic relationship where one can not exist with out the other. It partly for this reason that magic can be conducted using any sort of psychic energy because fundamentally, the psychic energy of the world is neutral.