[Kriyā yoga serves] to cultivate samādhi and to attenuate the afflictions.

This sutra explains the dual purpose of kriyā yoga: on one hand, it prepares the mind for deep states of absorption (samādhi); on the other, it weakens the kleśas, the afflictions that cause suffering.

Bhāvanā means cultivation, gradual development. Samādhi is not achieved suddenly; it is cultivated through sustained practice.

Tanūkaraṇa means to thin out, to attenuate. The afflictions do not disappear instantly, but they progressively weaken until they lose their power over us.

Kriyā yoga is thus a preparation: it clears the ground so that the seed of samādhi can germinate.