Over the years my sitting has evolved. As it is today I sit with my belly and ass somewhat out, shoulders back just slightly. Nothing exaggerated but nothing straining either. While I typically don't get excited during zazen, I have been known to get drowsy.
ops: I'm pretty sure my concentration has never been 'perfect'...
But for me, the sensation happens when I am relaxed, focused and my breathing has 'settled' as stated above.
A couple of additional things to note, my mouth is closed so I am breathing through my nose and I also breathe from my belly (lower diaphragm). This is a bit different than my normal day-to-day breathing which I find is up in my chest or at least split betwixt the two.