Kirtan is part of mantra yoga as well as bhakti yoga. Kirtan is joyful chanting in which the name of God is repeated. Kirtan is most powerful when it is done in a group. Together, a powerful vibration is generated in which the separation between individuals disappears. One turns towards God through dance, music and chants. Kirtan is a practice of devotion.
Kirtan is the first step of mantra as it is a tool of pratyahara (sense withdrawal), introverting the mind through external practice. When the mind and senses are still externalized, it is difficult to concentrate during mantra japa, and thus at the beginning kirtan is the most powerful practice.
However, it does not matter how far one is on the path of yoga, kirtan remains powerful as it is a tool to open the heart and connect with the higher consciousness. In kirtan, it does not matter whether one is a great singer or not. All what matters is the devotion to the Supreme. Bhakti is by itself a complete practice that can lead one to higher experience.
"Kumbhak ( Retention of breath) can be attained through Bhakti also. When the emotion reaches climax, the breath stops and the mind becomes fixed" – Neem Karoli Baba