Power generation without bending and tucking

Hi, Bruce, while I was watching your video on Tai Chi for Back Pain, I noticed that the original Yang form you learned had the body vertical, no bending/leaning forward, no tucking. Can you explain how you generate power in that case? Since that was pre-Yang Chengfu form, I would assume the older generations of Tai Chi masters generated power while the body is vertical without bending/tucking and hence I am highly interested in knowing the key points.

Also, can you compare the two ways, body vertical vs. bend and tuck, from the point of power and martial arts perspective?

Your help is highly appreciated.