Until you have signs that you have purified your negative actions and obscurations, it is important to be completely concentrated on the visualization and to do the recitation without distraction.

As the Great Master said,

If you are distracted, with your mind elsewhere,
Even reciting for a kalpa will bring no result.
What are the signs of success in this practice? Meditational experiences or dreams in which you vomit or purge, are washing, are dressed in white, cross a wide river, fly through the sky, see the sun and moon rising, and so forth, are signs that you have purified negative actions.

Dreams or experiences in which dirt, pus, blood, and lymph come out of your body are some of the indications that you have purified illnesses. Those in which minute animals such as ants emerge show that you have expelled negative forces. In particular, you may have real, direct experiences of clear awareness, of physical lightness, and of spontaneous devotion and determination to be free.

In general, if you have genuine confidence in the law of cause and effect, there is no way that you will not feel regret for your negative actions, and when this is so, your parting from faults too will be authentic. From purifying your mindstream in this way, you will certainly have more and more good experiences, and your realization will increase.

Nowadays, however, we are not fundamentally interested in purifying our obscurations and do not make the effort. Even if we do, we follow the prayers and rituals merely as a duty, but without real, heartfelt confidence or shame and regret.

This is why the qualities of experience and realization are as rare as stars in the daytime. So never underestimate the importance of this method for purifying obscurations, and make every effort to practice it so that it is truly effective.

Dudjom Rinpoche –
A Torch Lighting the Way to Freedom –
Shambhala Publications