The following excerpt from Yoga Beyond Belief by Ganga, is from chapter four, Finding the Ah Ha! In Hatha!  – Principles, Hints, and Insights into Yoga Practice. It comes from the section entitled: Concentration and Attention. This section teaches the difference between concentration and attention in practice.

"In the early stages of Hatha practice we have difficulty staying aware of everything required in a posture. There is breathing, specific alignment, movement of musculature, and energy. Concentration by its very nature has to move from point to point. Students often find that as they concentrate on one point, they lose another. The legs are energized and aligned and the arms go limp, for example. After some practice and progress, concentration begins expanding into a more all-encompassing attention. Be careful though: If attention is forced, it becomes concentration."