We often see or sense something that gives us a bit of a lift, or a moment's pure sadness. Perhaps it is the funnies flapping in the breeze before a newsstand on a sunny spring day. Or some scent on the wind catches us as we step from the bus, or bend to lift the groceries from the car (...) Haiku happen all the time, wherever there are people who are "in touch" with the world of their senses, and with their own feeling response to it. (Higginson, 3-4)
I think that a good haiku is a combination of impressionism, i.e., how we experience the event/apparition/scene itself in a "sensual" (through the senses) manner, and of suggestiveness, i.e., in not telling everything and allowing readers to respond the way they want.
Of course, the event/apparition/scene is also always suggested to us in the first place, and never imposed on us directly. As it is discreetly suggested to us, it is only fair that we should reciprocate by discreetly suggesting it to our readers.