Hi Daniel_G
I tend to view samatha-vipassana as a spectrum, with extreme points of pure solidified concentration at one end (samatha), and fast, moment-to-moment investigation (vipassana) at the other.
Here's my rule of thumb:
samatha: make the object solid, smooth, beautiful, bright, clear...
vipassana: take the object apart, look for small cracks, changes...
Concentration in samatha is continuous and restful.
Concentration is vipassana locks on moment-to-moment.
Another way to view this is: You build up concentration to be able to access a mental state, a set of mental qualities (samatha). Once your concentration is good enough to reliably enter that "place", you can look around and investigate, because not all your resources are bound up with staying there, learning your way, getting to know the place (vipassana). One day, this becomes routine, and you start climbing higher, and once again, almost all your concentration is bound up with just getting there. And so on.
Some examples with the breath: concentrating on a small area and being aware of in- and out-breaths, trying to make it smooth and nice, is almost pure samatha. This can take you to solidified jhana states (very nice!)
Concentrating on the breath, but quickly zooming in on the little suction when in-breath turns to out-breath, the little muscle movements and tremors as the body exhales and inhales, the vibrations of hair on the upper lip, and so on: vipassana.
Counting entire, smooth breaths: samatha. Quickly noting individual, changing breath sensations: vipassana.
The inner "nada" sound: resting on a continuous sound: samatha (my first taste of hard jhana, btw). listening for changing sounds: vipassana. Listening for quickly pulsating sound: vipassana. tuning in on a slow beat: samatha.
Just my take on the subject.
Cheers,
Florian
