Legendary Master Wu Chien Chuan demonstrated how to protect spine and back.

Legendary Master Wu Chien Chuan demonstrated how to protect the spine and back.  Youtube link

Grandmaster Wu Chien Chuan is doing a forward, push forward, leaning move. The picture with the red curved line, and the subsequent video, with a directional red arrow, shows a common posture that many Tai Chi practitioners use nowadays. This type of structure occurs when many practitioners attempt a “ forward push forward lean “ posture. 

Please, notice his body has a bend in the region of the Lumbar spine and hip joint area. So any stress or weight coming down will concentrate on that bent area. It is called a “stress concentration area”, There is such “stress concentration, especially if a student with such structure is doing sparring exercise and applying a force to the opponent or an opponent is applying a force to him. This additional force will all concentrate in the stress concentration area in that bend. So a wrong move like this could easily squeeze the disc from his Lumbar area and alter its position. 

Grandmaster Wu, his body structure is a straight line in this posture. He was actually stretching his body from his neck down to the Achilles heel of the back foot. That body is stretched, and the spine is also stretched so the force is coming down from his body. The force is transmitted down through the spine, through the leg to the heel, and absorbed by the ground.