An elaborate explanation of the underlying logic of this practice and the stages of the meditation can be found in texts such as my own Perfectly Illuminating the Profound Meaning: Advice on the Great Middle Way Beyond Extremes. Briefly however, the Intermediate Stages of Meditation says:

Having generated relative bodhichitta in such a way, we must exert ourselves so as to arouse absolute bodhichitta.

We must exert ourselves, as this says, in practising this profound path through study, contemplation and meditation, because it is certain that this alone will counteract the darkness of the two kinds of obscuration, and unless we overcome these two kinds of obscuration we can not possibly attain omniscience. As it says in the Introduction to the Bodhisattva’s Way of Life:

Emptiness is the antidote to the darkness

Of both emotional and cognitive obscurations,

So why would those who seek omniscience

Fail to meditate on it straightaway?[1]

We might wonder what is the nature of absolute bodhichitta, and how we cultivate it in meditation. The Intermediate Stages of Meditation says:

Absolute bodhichitta transcends the mundane; it is beyond all limitations; it is utterly clear; it is the absolute domain; it is stainless and unmoving, like a candle flame undisturbed by wind. To achieve it, we must devote ourselves for a long time to training in the practices of shamatha and vipashyana.

The Ornament to the Sutra Section also says:

Since there arises wisdom beyond any concept of things or events,

It is asserted to be the absolute [bodhichitta].[2]

As this says, its nature is the wisdom that directly realizes the natural simplicity beyond conceptual elaboration. It is realized through cultivating the samadhi that unites both shamatha and vipashyana. As the Introduction to the Bodhisattva’s Way of Life says:

Knowing that the destructive emotions are overcome

Through penetrating insight suffused with stable calm,

You should first seek the peace of calm abiding,

Which is found in joy and non-attachment to the world.[3]

The meditation on absolute bodhichitta is therefore divided into two parts: practice during the session, and practice in between sessions.