There are three ways of giving commentary. The first one is the guru explaining the commentary with practice; from his own experience of the practice. As it fits, the disciple can practice it. I think nowadays this is done in Dharamsala, India, and the Tibetan lamas also do it in Tibet.

First the lama gives the teaching on meditation to the disciples, then they go back to their rooms and sleep until the gong. I’m joking. They go back to their hermitage near the lama and try to complete the effortless experience of the meditation subject, such as perfect human rebirth, the eight freedoms and ten richnesses. Then they come back to check with the guru. If the disciple has generated the realization, the guru teaches the next meditation, then after having received the complete advice on that, he goes back to the retreat place, and so on.

The second way of giving commentary is called commentary with experience. After the disciples have the experience they check with the guru, they live close around him and all day they retreat on lam-rim. Then they check with the guru, who himself has the experience of the steps of the path to enlightenment and he sees whether they have generated the subject. If the disciple is making a mistake in the meditation, the guru clarifies this mistake and corrects the disciple’s understanding. So much happens for those who are making lifetime retreat on the lam-rim in Tibet.

via Practicing Lam-rim as a Householder by Kyabje Lama Zopa Rinpoche.