The timing is easier to understand if you know a little about plant metabolism.
Most plants produce new leaves and stem growth from some point in spring through midsummer.
As full summer heat sets in, the sugars are gradually transferred to the plant's woody parts and its roots, where they're stored during winter's dormant period. When spring arrives, the stored sugars are used to start new growth. For some plants, a combination of both late-dormancy and summer pruning often yields the best results. The following guidelines are most pertinent to climates with four distinct seasons and definite winter chill. In warmer-winter areas, timing will vary depending on the particular plant's native climate.