Ben wrote:But if you look at the commentary on anapana-sati in Vism, you'll see that Buddhaghosa talk about the quality of the concentrated mind. Its without physical or mental tension. Buddhaghosa says that one should place the mind on the object. Its relaxed, a gentle firmness of application. If you apply yourself in that manner you may find a better quality of concentration.
Tension, in what sense? Of course, emotional extremes like anger, fear, anxiety, sadness, desire, etc., disturb concentration, but the act of developing concentration can feel quite tense.
If this tension is entirely fabricated, is there no limitation on the immediate potential of concentration?
i.e., when trying to fully realize anatta, sunatta, or simply memorize something for school, there is tension when there is the inability to achieve a certain level of focus. Self-view cannot simply be focused away, rationality cannot be continuously maintained, suffering cannot be immediately seen through. Seeing through anger and ignorance is difficult.
As you mentioned: It's a "gentle firmness" of application. The "firmness" suggests at least some degree of tension, like using a muscle to lift something instead of snapping the muscle (in a punch or kick), a muscle spasm, or not moving it at all... the former two being metaphors for anxiety, the last one a metaphor for torpor...
Up to a certain point, it feels as though concentration can be "used up," that the ability to direct attention is like a currency with a fixed limit and you can't simply direct mindfulness outwardly in all directions infinitely.