How does bending over without bending knees stress your lower back?

The spinal column is a series of bones connected by ligaments and other relatively delicate tissues with a central spinal cord running through the middle and nerve roots coming out between the bones which go out to the rest of the body.

The easiest way to illustrate how it is strong when upright but weak when bent is to imagine a bunch of children's building blocks stacked up and being held together by strips of tissue paper glued to each side.

It can hold lots of weight pretty well when all the blocks are stacked neatly one on top of each other. But if you bend it and try to put weight on it, now the tissue paper takes a lot of the load and it can easily break apart.

When you lift while bending it stresses the various connective tissues on the outside of the vertebral column, especially the lamina of the inter-vertebral discs which are prone to tearing and then allowing the jelly like stuff in the middle to seep out and press on nerve roots.

Bending over without bending your knees works your muscles and if you're not flexible, pulls at them. That's why when you strengthen your core, most people are able to bend without bending their knees without pain.