There is another kind of concentration which does not depend upon restricting the range of awareness (that is, by fixing the mind upon a single object to the exclusion of other objects). This is called “momentary concentration” (khanika-samadhi).
To develop momentary concentration, the meditator does not deliberately attempt to exclude the multiplicity of phenomena from his field of attention. Instead, he simply directs mindfulness to the changing states of mind and body, noting any phenomenon that presents itself; the task is to maintain a continuous awareness of whatever enters the range of perception, clinging to nothing.
As he goes on with his noting, concentration becomes stronger moment after moment until it becomes established one-pointedly on the constantly changing stream of events. Despite the change in the object, the mental unification remains steady, and in time acquires a force capable of suppressing the hindrances to a degree equal to that of access concentration.
This fluid, mobile concentration is developed by the practice of the four foundations of mindfulness, taken up along the path of insight. When sufficiently strong, it issues in the breakthrough to the last stage of the path, the arising of wisdom.
Source: Based on “The Noble Eightfold Path: The Way to the End of Suffering”, by Bhikkhu Bodhi. Access to Insight (Legacy Edition), 30 November 2013, http://www.accesstoinsight.org/lib/authors/bodhi/waytoend.html