From my understanding of Leigh Brasington's teachings on (sutta) jhanas, you focus on the pleasant sensation and it will create a feedback loop effect until you can feel rapture and pleasure. Then you try to spread these sensations throughout the body. But you can focus on the pleasant sensation with the help of the breath. Just as in the anapanasati sutta it says that you breathe in sensitive to the entire body, you can use the trick of breathing in sensitive to the pleasant sensation, breathing out sensitive to the pleasant sensation in order to not lose the breath. It becomes like the bass, or drums in a rock music in the sense that it's not the main theme of concentration, but it marks the rithm and qualities of the meditation.
I hope I'm not butchering Leigh Brasigton's teachings.
He turns his mind away from those phenomena, and having done so, inclines his mind to the property of deathlessness: 'This is peace, this is exquisite — the resolution of all fabrications; the relinquishment of all acquisitions; the ending of craving; dispassion; cessation; Unbinding.'
(Jhana Sutta - Thanissaro Bhikkhu translation)