Hmm. This is quite interesting.
When I first read about first breathing aware of long, then aware of short, this struck me at first as odd and “backwards”. And then I tried it and it actually worked, much to my surprise.
Having practiced for decades counting breaths, I was was basically accustomed to just breathing aware of long/light. And the breath did indeed get gradually longer. But it required the constant effort of counting and diligently trying to lengthen the breath, so it was never quite natural. I was therefore curious to try this new method of breathing aware long/light, followed by breathing aware short/heavy.
And sure enough, I soon became aware of the usual long breaths. However, after some time I realized that some of these long breaths were indeed shorter than the others (!). So I became aware of them as short and slightly heavier. What was odd was that this dual awareness was effortless and yet it naturally shifted breathing towards long and completely got rid of “trying to make the breath longer”. This one little awareness of a short and slightly heavier breath actually lengthened the breath with no trying on my part. In other words, the sequence of long/light aware followed by short/heavy aware worked when I tried it for the first time this year. It took less effort and was smooth and quick.
Unless I’ve got it wrong. Maybe that’s just an unusual experience.