According to Ven. Gunaratana: (ref: http://www.accesstoinsight.org/lib/auth ... html#ch3.2
"Sustained Thought (vicara)
Vicara seems to represent a more developed phase of the thought process than vitakka. The commentaries explain that it has the characteristic of "continued pressure" on the object (Vim. 142; PP.148). Applied thought is described as the first impact of the mind on the object, the gross inceptive phase of thought; sustained thought is described as the act of anchoring the mind on the object, the subtle phase of continued mental pressure. Buddhaghosa illustrates the difference between the two with a series of similes. Applied thought is like striking a bell, sustained thought like the ringing; applied thought is like a bee's flying towards a flower, sustained thought like its buzzing around the flower; applied thought is like a compass pin that stays fixed to the center of a circle, sustained thought like the pin that revolves around (Vism. 142-43; PP.148-49).
These similes make it clear that applied thought and sustained thought functionally associated, perform different tasks. Applied thought brings the mind to the object, sustained thought fixes and anchors it there. Applied thought focuses the mind on the object, sustained thought examines and inspects what is focused on. Applied thought brings a deepening of concentration by again and again leading the mind back to the same object, sustained thought sustains the concentration achieved by keeping the mind anchored on that object."