Through the union of apāna and prāṇa, the decrease of urine and feces, even the old becomes young through constant practice of Mūla Bandha.

This verse describes the effects of Mūla Bandha:

1. Apāna-prāṇayor aikyam — The union of apāna and prāṇa. This is the main subtle effect: the two opposite pranic currents converge.

2. Kṣayo mūtra-purīṣayoḥ — Decrease of urine and feces. Indicates reduction of elimination processes, interpreted as conservation of vital energy.

3. Yuvā bhavati vṛddho ‘pi — The old becomes young. Rejuvenation.

The union of prāṇa and apāna is a central concept in haṭha yoga. Normally:

• Prāṇa rises (heart, head)
• Apāna descends (elimination, sexuality)

This polarity keeps energy dispersed. When they are unified through bandhas, the concentrated energy awakens kuṇḍalinī and ascends through suṣumnā.

Satatam — constantly. The benefits require regular and sustained practice, not sporadic efforts.