Interpretation of "atha yoganusasanam"
Direct translation: With prayers for divine blessings, now begins an exposition of the sacred art of yoga. Now follows a detailed exposition of the discipline of yoga, given step by step in the right order, and with proper direction for self-alignment
(p.44 “Light on the Yoga Sutras of Patanjali” by BKS Iyengar)
“Now begins” is also thought to be “only when…is the time …come true” because atha has meaning of auspiciousness and a good omen. “Sacred art of yoga” also means the way to reach its final stage; Samadhi, to transcend your consciousness to unite ultimate. “Self-alignment” is the state that your mind, body and soul are merged into one.
Hence the interpretation can be “This is the exact precise guide for yoga, that is the state the one to merge his/her body, mind and soul into one in order to transcend his/her consciousness to merge with Supreme Spirit. And it will come true only when his/her consciousness is here with divine blessings.