What is initial application (vitarka)? What is sustained examination (vicāra)?

Vitarka (initial application) is searching without a target, like shining a flashlight around everywhere—its movement is obvious and rapid. Vicāra (sustained examination) is when, after searching everywhere, one has found a rough target, and so attention is focused on the target for observation; its movement is light and slow.

When investigating Chan, after one has found a rough direction and focus for the investigation, one begins to lock onto a target for observation and scrutiny; the mind becomes even more subtle and dense, and is difficult to detect.