When performing lifts which require pulling weight off the ground (i.e. deadlift, pulls, snatches, cleans etc.), you need to keep your back tight and use your legs to move the weight off the ground. Your back muscles are stabilizing muscles to support your spine and prevent injury. No movement should occur in your back and it should be solid throughout the lift. This came to my mind as it is still something I am working hard on to achieve my 200kg total.
Picture from USA Weightlifting of Natalie Burgener.