Robin Woods:
I don't mean to coarsen the nature of debate around here, but, is the 'piti' felt at the threshold of the 1st jhana often experienced as as a tingling sensation in the, ummm, genitals?
Well, that's going mighty low on the physical chart to try to locate
piti (rapture/elation). Is that how you view the experience of
piti? As a physical sensation that could be placed anywhere in the body? That seems to be a bit of a wide variance, perhaps above and beyond that suggest in the discourses.
Piti and
sukkha are generally characterized (according to my understanding and view from having read the suttas) as mental phenomena. Placing them anywhere other than in the region of the head (as in a pleasant "alpha sensation") may be problematical. Or at the very worse, ingeniously inventive! (Sorry, but I don't agree with Laurel here. You might be better off focusing on the head region rather than on any other region of the body. It's all about control, once you understand how these processes arise and display.)
Although, if you can make it work, who knows. . . ?
Robin Woods:
What does one do when noticing it? Leave the breath and focus on the tingling in the hope that it will expand?
No. Follow the breath always, but note the arising pleasant sensation and just watch where it takes you as the first jhana morphs into the second jhana which is without effort or toil. In other words, be aware of and watch both together, maintaining primary focus on the breath while not discounting the arising sensation.
The second jhana "feels" like a feedback loop that just carries on by itself. When there's no longer any need for
vitakka and
vicara (directed attention and sustained attention), then the absorption process proceeds on its own without the need to manufacture it.
Until you experience this, you may not know what I'm talking about.
Good luck.