These few days, I've been doing the Diaphragmatic breathing fast, over 30 breaths per minute, for 10 minutes. Today, I did it for 15 minutes, and there were some differences. On and off, in the first few minutes, there were warmth spreading in many parts of the body, plus the (usual) expanding/contracting energy in some places, mainly the back.That is, an energy field surrounding the body that is expanding/contracting. Seems like on of Shinzen Young's meditation methods. For the last 3 minutes or so, vibrations settled in, in the torso and mouth. Didn't do it longer just to play safe.
When I stopped these breathing, I didn't need breath for around a minute. As there weren't jhanic territory to explore like in previous sessions, I just watched my torso to see how the first new breaths would be. The first one was very short and from the dantien (4 fingers below the navel). The second (30 seconds later) was above the navel and the third (~30 secs too) was deep and involved more surface. Then, during the 50 minutes sit and for around 30 minutes off cushion, the diaphragm kept relaxed, really relaxed.
Regarding the first breathing, it reminded me some old experiments I did with taoist reverse breathing, contracting the lower belly while in-breathing. Despite there's always some (subtle) tension in the torso even after out-breathing, if you just wait enough, the body will inhale contracting that way. Try this: sit in your couch with your legs/crotch open, resting above a coffee table or chair. Be sure that you are at an angle, so that your tailbone is tuck in. While resting comfortably there for a while (maybe watching TV), you'll see that right before the inhale there's a gently pull in of the abdomen in the lower belly.
Perhaps I should add in the future the Bhastrika breathing.
Added: diaphragmatic breathing