"Ponder the fact that someone realized the Way by hearing the sound of bamboo; that another clarified the Mind at the sight of peach blossoms," Zen Master Dogen said on several occasions, or some variation thereof.  In all my time practicing Zen, in all of my reading, and in all of my conversations with fellow practioners, I never heard of anyone realizing the Way, awakening, while sitting in zazen.  No one's ever fallen off the cushion exclaiming, "I get it!  I get it!," or for that matter levitated off of the cushion, either.

Here's what I have seen:  people experience these moments of insight, they have these awakening moments, when they least expect it, but during a time in their life when they're practicing a lot of sitting meditation.  After hours or days of stillness, someone speaks a phrase ("the mind that sticks to nothing"),or a swept pebble hits bamboo, and suddenly, there is realization of the Way.  But the quiet and stillness of zazen is silent and still - it is not the realm of activity, including awakening.

Everything arises from conditions.  The pebble does not strike the bamboo on its own accord - there is sweeping involved, not to mention the 10,000 things that caused that particular pebble to be in the path of that particular broom at that particular time.  But there are also the conditions in the mind of the one who hears it, and the quiet and stillness of zazen conditions the mind to be receptive to the arising of the Way.

Nangaku chided Baso for practicing zazen with the intention of becoming a Buddha, but Baso's actions were at the same time the planting of the karmic seeds that would eventually bring him to his Buddha-nature.  Nangaku's chiding was the sun and the moisture and the warmth that helped those seeds reach fruition.