As far as explanations go, they’ve got it quite right 
It’s only slightly abbreviated version of MN118:
It’s when a mendicant has gone to a wilderness, or to the root of a tree, or to an empty hut. They sit down cross-legged, with their body straight, and establish mindfulness right there. Just mindful, they breathe in. Mindful, they breathe out.
When breathing in heavily they know: ‘I’m breathing in heavily.’ When breathing out heavily they know: ‘I’m breathing out heavily.’ When breathing in lightly they know: ‘I’m breathing in lightly.’ When breathing out lightly they know: ‘I’m breathing out lightly.’ They practice breathing in experiencing the whole body. They practice breathing out experiencing the whole body. They practice breathing in stilling the body’s motion. They practice breathing out stilling the body’s motion.
They practice breathing in experiencing rapture. They practice breathing out experiencing rapture. They practice breathing in experiencing bliss. They practice breathing out experiencing bliss. They practice breathing in experiencing these emotions. They practice breathing out experiencing these emotions. They practice breathing in stilling these emotions. They practice breathing out stilling these emotions.