What happens if you decide not to concentrate, stop thoughts, or meditate in any way, but instead simply take 30 minutes to enjoy some deep abdominal breathing?

What if you notice the intention to pay attention to the abdomin as a sensation?

What if you drop any intention to breathe and let the next inbreath begin by itself?

Answering your questions, I haven't found tranquility with those ideas.

Try this: After you exhale, do not breathe by yourself. Let the inbreath begin by itself and then just pay attention to it when it begins.

How do you avoid the watchdog effect, where it feels like you have to pay attention? This creates a background tension that I've noticed lately - like a feeling of contriving where I focus. If I set the intention to "breath deeply" instead, the attention seems to rest more naturally on the breath and I don't have to think about it.

Excellent!

Good to even do a few throughout the day, to stay grounded.  I think this releases alot of stored up tensions, this post is a good reminder, thank you.

Reminds me of the Anapanasati Sutta also, but a translation I have always questioned:
[1] Breathing in long, he discerns, 'I am breathing in long'; or breathing out long, he discerns, 'I am breathing out long.' [2] Or breathing in short, he discerns, 'I am breathing in short'; or breathing out short, he discerns, 'I am breathing out short.'


Perhaps it could be read thus, and it might make more sense:


[1] Breathing in deep, he discerns, 'I am breathing in deep'; or breathing out deep, he discerns, 'I am breathing out deep.' [2] Or breathing in lightly, he discerns, 'I am breathing in lightly'; or breathing out lightly, he discerns, 'I am breathing out lightly.'

In this way, it could be understood, that when one pracitices deep breathing (such as deep abdominal breathing), then this naturally calms the body/mind, which leads to a lighter, more shallow state of breathing, and hence a mind more at ease.

Now don't get me wrong, I am also a proponent of Pranyama, and other sorts of Energy breath circulation techniques and methods, It may sound as if I am strictly a Buddhist from my posts, but this is not the case, just that other subjects usually don't come up that I feel versed enough to talk about or explain.  And usually the Buddha explained things so succinctly and straightforward, I can't really find any holes in his teachings, except he didn't teach about a vast number of other subjects.  Anyway, some links...

Would also be curious about how these methods may make make a union of the body/mind through the Vagus Nerve....  As do other techniques.

Deep Belly Breathing, also called Diaphragmatic Breathing, among other names/other traditions, I know Reggie Ray teaches this

http://www.swamij.com/diaphragmatic-breathing.htm



http://www.health.harvard.edu/newsletters/Harvard_Mental_Health_Letter/2009/May/Take-a-deep-breath


Psi

Watchdog effect doesn't happen to me because there is just abdominal muscles moving and the sensation of the rest of the body.

When the watchdog effect happens to you, are you trying to pay attention to just the abdominal muscles? If yes, try to be attentive of the whole body and that would obviously include the rising and falling of the abdomen. I think that should take care of the watchdog effect.

If it persists, then I guess that you can't include your head while trying to be attentive of the whole body. To remedy this, you will have to do some stretching exercises/yoga poses.

Doesn't the intention to breath deeply create tension too, aswell as enjoying it for all of the decided time? Sensual clingning, isn't that a hindrance?

Enjoyment isn't clinging - the desire to keep enjoying is the clinging.