To someone new to deciduous shrubs, whacking and hacking branches from a plant seems likely to injure it, not to help it blossom and flourish.
However, an annual cut for most of these shrubs is a good thing -- as long as it's done right.
If a branch fills the jaws of a hand pruner, you should be using a lopper. If it fills the lopper jaws, bring out the saw.
Timing is key to pruning a deciduous shrub.
For lush growth, spring-flowering shrubs require trimming as soon as the blooms fade. Don't leave a stub. Make the cut as cleanly as possible against the remaining branch. On younger shrubs, prune branches back to nonflowering shoots or back to healthy buds pointing in the direction you want the shrub to grow -- typically away from the center. Always cut back diseased or weak stems to healthy wood. Disinfect the blades with a 10 percent solution of bleach between cuts to prevent spreading disease.
Young, newly planted shrubs, especially witch hazel (Hamamelis spp.) and magnolia, require cutting back after their first bloom. The trim helps them to grow a stronger framework and bloom more profusely later. Some species, such as Forsythia Y intermedia or beautybush (Kolkwitzia amabilis), may not bloom heavily their first few years, but they still need weaker growth and wayward branches cut out; their main shoots should be trimmed to a strong bud or pair of buds.
Older, established shrubs occasionally need reshaping and removal of dead or thick stems to look attractive. A neglected, overgrown shrub may revive if old, woody stems are cut down to the base allowing younger stems to take over -- but this may delay flowering for a year.
Some species are exceptions to the rule.
For instance, mophead and lacecap hydrangeas (Hydrangea macrophylla) bloom from shoots formed the previous growing season, so they prefer minimal trimming. Other shrubs that bloom in late summer or autumn, such as butterfly bush (Buddleia davidii), favor trimming back in early to midspring for the best bloom in fall. Pruning late-blooming shrubs after they flower will also leave them susceptible to frost damage. Check with your county extension office or your local nursery's shrub expert about the best time to prune a specific species.