wingchun wrote: this sounds like what i was referring to, are you referring to engaging your transverse abdominis (pull your belly button back towards your spine) while breathing so that your rib-cage expands?
That's a similar exercise for lower dan tien. The exercise I was talking of is a similar kind of thing but aimed at the middle dan tien. Once you can do middle and lower, you can do both at the same time activating the transverse abdominis and the latissimus dorsi muscles so your lower and upper back expands outwards and back in a curve.
In my line of ziranmen we start these exercises as a breath method. When done correctly the breath method alone will stretch your back muscles and open them up a bit. After the breath method we progress to a series of whole body stretches based on self-resistance which use the breath method as part of the stretch. To be honest I'd have to do a video to explain these properly.
From a martial aspect the training can be used facilitate a peng-jin like power (to use a taiji term) but if you expand your body in this way quickly enough you can also use it to develop a one-inch punch type power. In application you throw the punch and then add this type of expansion just on impact.
Use of this type of power is not restricted ziranmen. I've seen similar concepts in taiji and xing yi. Some people in taiji talk of holding zhan zhaung posture and imagining a feeling as if they are holding an ball that is inflating so their arms are expanding outwards in all directions but their fingers end up pushing towards each other in a pincher movement because it's as if their arms are becoming elongated. I'm not sure if you can makes sense of that description but its the same kind of concept. I don't know if these other systems have exercises to train this power in this specific manner though.