Kmath, on what led me to believe this:
Observing my own practice over short and long term, being mindful of what produced piti sukha, what nourished it and what types of conditions could vary the magnitude predictably. For example, if I'm doing 6 hours of sitting meditation a day and no other activities that are energy depleting (labor intensive chores, talking to people, etc), then pitisukha tends to be very mild, because all the batteries (every cell in the body) is pretty much fully charged. What I feel in these meditations are just very comfortable, unusually comfortable but subtle piti sukha. The body feels light, strong, quick. If I do for example one 1-hour sit in the morning, followed by say 4-6 hours of energy depleting activity, and then do a 1-hour sit in the later afternoon, then I get much more intense piti-sukha closer to showering rapture end of the spectrum, it feels like all the batteries are getting charged and the brain is getting endorphins to discourage you from ending the meditation prematurely until it's charged again. But once I'm all charged up piti-sukha drops off again.
The other way I can get piti-sukha to show up in small localized patches is if I stretch (muscles,tendons,ligaments), do qigong, acupressure on my self, and any places that were tight and opened up a bit, then I feel localized piti-sukha in those areas. So places that are clear and free flowing energy, zero to very little local piti-sukha, but places that were tight and now opening up due to stretching/exercise etc, get a charge of piti-sukha.
When my microcosmic orbit first opened up, I had been stuck around the head/throat area for years, when it first opened up it felt like a plugged up bathtub had it's clogged drain opened just slightly, and I could feel a sensation of a small thread of water flowing down from the throat to the dan tien (abdominal) area. As the front side of the channel opened, then piti-sukha got to be intense. At times it felt like an orgasm was revolving along the path of the microcosmic, and particularly intense in areas that were tightest. While doing anapanasati, if I focused my spatial attention on a hot spot (that was formerly tight) I could get a mini to strong orgasm localized to that area. Different meditators open up differently, mine was more piecemeal, but I could understand how some meditators feel like their entire body is having orgasm. That's the more extreme end of the spectrum. As the front side of the microcosmic opened up, it felt like a highway that was being expanded to 2 lanes, 4 lanes, etc, and then the 1 revolution around that formerly took several minutes, as the channel cleared started approaching 1 inhale/exhale cycle per revolution.
There's a reason the buddha uses water in the jhaana similes for 1-3, because often the piti-sukha feels like water flow. But also heat, electricity, comfortable vibrations.