These are simply large long-handled clippers for a larger job. The plants appreciate the clean cut of the proper tool. You will, too. The right tools make any job easier. When you cut off a part, the growing energy that went into that part goes to the remainder of the plant, producing new, lush growth.
Timing is important for flowering shrubs.
Those that flower in early spring should be pruned as soon as the flowers have gone by. The plant will have the entire season to put on new leaves and buds, which will flower the following spring. Forsythia, lilacs, and azaleas are examples of this group. Late season bloomers flower on the growth of that year. Prune them when they are dormant in late winter or very early spring.