However, because the aptitudes of meditators differ, several suttas allow for alternative approaches to mental cultivation: 1) The first approach, the classical one, is to develop serenity first and insight afterwards. By "serenity" it is meant the jhanas or (according to the Pali commentaries) a state bordering on the jhanas called "access" or "threshold" concentration (upacarasamadhi). [...] The description of the fourth approach is somewhat obscure. The sutta says that a monk's mind is seized by agitation about the teachings, and then some time later he gains concentration and attains the supramundane path. The statement suggests a person initially driven by such intense desire to understand the Dhamma that he or she can not focus clearly upon any meditation object. Later, with the aid of certain supporting conditions, this person manages to subdue the mind, gain concentration, and attain the supramundane path.
"Or again, friends, a monk's mind is seized by agitation about the teaching.11 But there comes a time when his mind becomes internally steadied, composed, unified, and concentrated; the path arises in him. He now pursues, develops, and cultivates the path, and while he is doing so the fetters are abandoned and the underlying tendencies eliminated. "Friends, whatever monks or nuns declare before me that they have attained the final knowledge of arahantship, all these do so in one of these four ways."
Dhammuddhaccaviggahitam manasam hoti. Mp says that "agitation" (uddhacca) arises here as a reaction to the ten "corruptions of insight" (vipassanaupakkilesa) that one misunderstands as indicating path attainment (On the corruptions of insight, see Vism 663-38; Ppn 20:105-28.) It is possible, however, that the "agitation about the teaching" is mental distress brought on by eagerness to realize the Dhamma. This state of spiritual anxiety, when suddenly resolved, can sometimes precipitate an instantaneous experience of awakening. For an example, see the story of Bahiya Daruciraya at Ud 1:10.