कनीयसि भवेद्स्वेद कम्पो भवति मध्यमे । उत्तमे स्थानं आप्नोति ततो वायुं निबन्धयेत् ॥ २.१२ ॥

kanīyasi bhavedsveda kampo bhavati madhyame । uttame sthānaṃ āpnoti tato vāyuṃ nibandhayet ॥ 2.12 ॥

In the beginning stage of pranayama there is perspiration, in the middle stage there is quivering, and in the perfect stage of pranayama one obtains superiority (the pranavayu reaches the brain), and that is why Kumbhaka should be practiced.