Austerity, self-study and surrender to God constitute Kriya Yoga.
The commencing chapter of the Yoga Sutras is on Cognitive Absorption or Samadhi, the summum bonum experience-wise of what it is to be a Divine Being having a human experience. The succeeding chapter, chapter 2 of the Yoga Sutras is on Spiritual Practice to facilitate the path to Samadhi and Final Liberation or Kaivalya.
Rishi Patanjali begins the chapter with a condensed formula for Self-Realization (i.e. realizing the Self within self and all selves). He calls it Kriya Yoga or Union through action. It can be achieved by practicing Tapas (austerity/asceticism) which means restraint of body, mind and senses through self-control in order to ultimately pierce through their veils, Svadhyaya (self-study) equating to ‘Man know Thyself’ through both the study of Holy Texts (Scriptures) but also of one’s innate psychology with the objective of transforming one’s base nature to reflect a/the Higher Self, and Ishvara Pranidhana (surrender to God/the Lord) being devotion to the Highest Ideal inherent in the Universe to become Divine (or reclaim one’s Divine origin) and perceive Divinity in all beings.