Chapter 1, Sutra 20: "The samadhi of (some) others has as its preconditions faith, strength, memory, meditation and the awakening of wisdom in samprajnata." Swami Veda Bharati
Explanation: For those for whom samadhi is not naturally experienced at rebirth, they may attain samadhi through effort, by Faith (shraddha), Strength (virya), memory (smriti), concentration, and development of wisdom (prajña).
Faith protects the aspirant from doubt, creates tranquility, and engenders determination to reach the goal and pass through the obstacles along the way. Strength enables the application of energy, enthusiasm, and courage to move forward. Memory allows to aspirant to continually focus on the processes and the goal, and avoid past mistakes. Concentration brings the full power of the mind to bear; it is like concentrating sunlight through a magnifying glass. Normal sunlight might illuminate paper while concentrated sunlight burns through. Wisdom is the ability to discriminate between the Real and false self.