Explain A?

This is not a must be true, we are not concluding A, but rather, we are using A to make the argument valid. The argument holds that since one rake is better than the other in terms of compression stress, it is better in terms of reducing spinal injury. This does not have to be true, as far as I know, compression stress is just one of many types of things that can cause spinal injury. A fixes this flaw by equating the two elements.