When the meditator purifies the mind of the nine lesser defilements by attending to the in and out
breaths, he obtains the arising of the sign.
“The sign”: It is like the pleasant touch of spinning silk or spinning cotton wool touching the
body or it is like the pleasant touch of a cool breeze touching the body.
Thus, contemplation of the touching of wind of the in and out breaths at the nose or on the lip
gives rise to the sign of wind, which is not is due to shape or colour.This is called “the sign.”
If the meditator practises and much practises the sign, he can increase it at the nose-tip, at the
glabella, on the forehead or he can establish it in many places. He pervades his head with wind.
Through increasing in this way he can pervade his whole body with the happiness of tranquillity
(passaddhi-sukha). This is called “fulfilment.”
Again, there is a meditator who sees deviant signs from the beginning such as smoke, mist, dust,
gold sand, or [he experiences] the pricking of a needle or an ant’s bite, or he sees various
colours/forms (vanna, rúpa). If the meditator's mind does not clearly comprehend these deviant signs,
his mind will give rise to deviant perceptions, and there is distortion [of perception] (vipallása,
vipariyáya), not the perception of breathing.
If [however] the meditator clearly comprehends, he does not attend to strange signs. When
mindfully breathing in and mindfully breathing out, he prevents the arising of other perceptions. If he
attends thus, deviant signs promptly cease, he acquires the subtle sign940 and his mind becomes diligent
(appamáda). - Vimuttimagga
.
When you see that a nimitta has appeared, mindfully focus your awareness on it — but be sure to focus on only one at a time, choosing whichever one is most comfortable. Once you've got hold of it, expand it so that it's as large as your head. The bright white nimitta is useful to the body and mind: It's a pure breath that can cleanse the blood in the body, reducing or eliminating feelings of physical pain.
When you have this white light as large as the head, bring it down to The Fifth Base, the center of the chest. Once it's firmly settled, let it spread out to fill the chest. Make this breath as white and as bright as possible, and then let both the breath and the light spread throughout the body, out to every pore, until different parts of the body appear on their own as pictures. If you don't want the pictures, take two or three long breaths and they'll disappear. Keep your awareness still and expansive. Don't let it latch onto or be affected by any nimitta that may happen to pass into the brightness of the breath. Keep careful watch over the mind. Keep it one. Keep it intent on a single preoccupation, the refined breath, letting this refined breath suffuse the entire body.- Keeping the breath in Mind- Method 1, Ajahn Lee Dhammadharo