When one cultivates strong concentration by means of tranquility or insight meditation, the mind is no longer distracted by thoughts and other hindra… - Mahasi Sayadaw

The Vammika Sutta likens skeptical doubt to a fork in the road. Say a traveler who is carrying many valuables arrives at a fork in the road; if he lingers there unable to decide which way to take, robbers may catch and possibly kill him. In the same way, a doubtful meditator who falls prey to wavering and procrastination cannot continue on with practice. He or she will then become a victim of mental defilements and be unable to escape the cycle of suffering. Only when he or she abandons doubt by noting it and uninterruptedly continues the practice can he or she be liberated from the cycle of suffering.

Momentary concentration that focuses on mental or physical objects from moment to moment is called nondistraction (avikkhepa), a mental state that is the opposite of restlessness. When momentary concentration becomes strong, the mind seems to penetrate its object every time an object is noted.

Sometimes the mind may wonder about practice itself. For example, the mind may wander off into analyzing whether an object was noted effectly, or sequentially, or vividly. The mind may wander off into thinking about how one is going to note an object. Under these circumstances, insight concentration cannot arise.