Perhaps this was already suggested in the discussions about what is sometimes known as EiS jhana or sutra jhana, but I think I've found a suitable explanation of the cause of the two very different types of concentration I've experienced. In the past week as I've started working with the breath again I've noticed that it is way more pleasant, I can sit for about 3x as long, and it is causing a lessening of hindrance-creation in daily life.
I think that the essential difference is the way in which one pays attention to the breath. I'll start with a metaphor. It's as if your attention was a piece of paper, the object was a table outside, and the hindrances were blowing winds. In the first type of jhana I've practiced, I would just hold the piece of paper against the table, establishing a continuous intention or a quickly repeating one, concentration increased as I pressed harder and it began to sort of "lock in" with jhana.
With what I am doing now I simply lightly place the paper on the table and I back off. Letting it rest their as best it can by it's own inertia. It will get blown away, and I will watch, knowing that I could have/could be holding it down, then I chase it down and lightly place it on the table again. For whatever reason, (I am not a weatherman) the winds slowly die down and the paper just rests there effortlessly.
Each time you bring attention back to the breath there should be a sense of relief, not effort, and no part of your body should tense up. With this type of jhana, discernment happens naturally (of the understanding fabrication variety) because you are repeatedly letting go and learning about the hindrances rather than tightly holding and not really learning about anything. Concentration develops out of discernment rather than will, you discern the drawbacks of hindrances repeatedly.
