Pīti in the jhānas is equivalent to pīti-sambojjhanga (awakening factor). If you study SN 46.2 and 46.3, it will give you a pretty good summary of what that’s about. If you do an exhaustive search of pīti in the EBT, you’ll find in the context of 7sb (awakening factors), it often is qualified as Pīti manassa (mental). And if you look at what the activity going on for the Pīti manassa, it’s usually something close to being happy that your’re purifying your mind, activities, that is blameless, leads to your own welfare, that of others, and you can practice day and night (that’s how it ties in the previous bojjhanga of viriya).
So primarily piti in jhana and sambojjhanga the Buddha is emphasizing the mental development, what you do that causes/triggers a kusala mental happiness. But in meditiative practice, just like smiling, the mental cause is going to instantaneously trigger a physical response. A smile loosens up the tissue around your chest and heart, you may feel a surge of warmth, goosebumps, pleasant tingling up and down your spine, etc.
So In short, Ven. Thanissaro just concludes that Pīti and sukha both have physical and mental components. But the more detailed answer is that piti sambojjhanga and piti as a jhana factor, primarily we’re training the mind to trigger mental happiness by reflecting on our virtuous progress. There are other kusala ways of triggering Pīti, in SN 46.2 the Buddha punts on that question and just says “there are many ways” without being specific, so that leaves the possibility one could use a physical trigger instead of a mental one to get the process started. But I’m not aware of an explicit instruction in the EBT. Whereas the mental trigger of reflecting on virtue is frequent.
I was just reading Vedana Samyutta last night to see if sukha vedana is primarily physical or mental, and several suttas in there indicate the physical is the primary default, when you see vedana in the 3fold classification. Under the 5 fold classification as I discussed in the previous post, somanassa and domanassa would be mental and the sukha and dukkha would be physical.
The Buddha goes out of his way for passaddhi and sukha in the jhanas to emphasize the physical part as well, just to be absolutely clear. But in actual meditation practice, you’ll find that sukha as a physical trigger is likely to induce a mental pleasant reaction immediately as well.
So for 3rd jhana when piti drops out, it’s like the emotional thrill aspect of 1st and 2nd jhana wears off and you don’t get excited by it anymore. the sukha in 3rd jhana is just a pleasant physical comfort, maybe a smooth mental comfort accompanies that as well. When that sukha of 3rd jhana wears off, in 4th jhana you know your body feels light, soft, comfortable, but you feel indfferent towards it, it’s just heat, water, air element, not uncomfortable, not comfortable.
“Rapture” was probably intended to mean the emotional thrill aspect of pīti. To use a crude and inappropriate simile, it’s like a supermodel was out of your league likes you and marries you, and for a few months you have the emotional thrill (rapture) that, “wow! how could this happen to me!” But after those few months you just get used to it and take it for granted. That’s piti dropping out, and sukha is the smoother happiness of still sort of appreciating your good luck.
A more appropriate simile, I think this comes from Vism. and does not appear in the EBT, but is actually a really accurate simile in terms of emphasizing the mental and physical aspects in the right spots, goes something like this:
You’re stuck in the desert dying of thirst. Then you see ahead of you an oasis with cool drinking water. The thrill of knowing you’re not going to die of thirst and are going to taste sweet water shortly, is pīti, the emotional thrill. When you get to the oasis, jump in there splash around and physically drink the water and taste and feel the cool refreshing feeling of that, that’s sukha.