When we are in tadasana, or any pose when we are trying to extend the torso up to the heavens, what muscles are we working? Are we actually just taking the erector spinea muscles etc? into their full expression of length? some people can grow so much taller when focusing on that movement and it feels like a lot of work is happening.
Diaphragm and erectors are working.
How are the erectors working? Are they pulling up from a vertebra above, or just coming into their full length?
Because there are 6 layers of these muscles, different layers respond to different motions. The more superficial the muscle the more elongation it can bring the spine. The deeper muscles are shorter and respond only to subtle motions.