Getting injured is the number one way that you can fail to transform your body. There are some injuries that are purely unavoidable, and there are some that we can prevent if we follow basic principles.
All movement within a joint (or exercise) can be visualized as a circle. When performing resistance training, we are essentially strengthening our body through a specific range of movement. Our bodies are highly adaptive and will get better at whatever position you get it into - there are no bad exercises (of course there are some better exercises than others).
We need to identify a range of motion within our total range of motion that we want to strengthen. Visualise that as a triangle within the circle. The triangle makes up 3 principles:
The Acute-to-Chronic ratio looks at the difference between your average load to your intra-workout load. If you normally squat 40kg, and the next day you try to lift 60kg, you are putting yourself at risk of injury (intensity ratio). If you normally do 10 sets of leg workouts, and then you decide to do 20, you are putting yourself at risk of injury (volume ratio).
Acute changes in force, such as bouncing out of a squat, or rapidly changing direction will increase your risk of injury. Adding a load to the equation will also increase the chance of injury - it's basic physics. If you apply an acute load to a structure that is not adapted to it, you increase your chance of that structure failing.
Clear start and endpoint is not referring to an obvious stop at each end of the range. It's best to have a clear definition of your start and endpoint.
We want every rep to look consistent. If you watch yourself exercising, you should have some consistency between reps. There is a risk of obsessing on making your form perfect. If you complete the same rep, the exact same way for years, your body will not be able to handle the load when you step out of your trained range of motion. The internal forces will be different with every single rep.
If you're a regular gym-goer trying to improve your health, it probably won't matter too much to you, but if you're an Olympic athlete, we need to have some variance in our technique so that our body can adapt and handle those times when we step outside of our trained range of motion.