Correct form while exercising follows a principle of joint by joint mobility vs. stability. Mobility is the ability to move a specific joint through a full range of motion for an activity. Stability is the ability to restrict movement at specific joints during an activity. As we perform exercises it is important to pay attention to which joints should be moving, and which should remain stable. If the incorrect joints are mobile or stable you put yourself at a higher risk of injury, and a greater chances you are not targeting the correct muscles.
The picture above shows exactly which joints should be stable vs. mobile. The head, shoulder, thoracic spine, wrist, hip and ankle should all be mobile joints. If you have a hard time moving these joints you will benefit from corrective mobility exercises to allow more freedom of movement. The neck, shoulder blade, elbow, lumbar spine, knees, and ankle should all be stabile joints. This means that although these joints can move, the motions are controlled and support the movement of mobile joints. If you are unstable in any of these joints, corrective stability exercises will benefit your physical performance. Corrective exercises and the concepts behind them are not always easily understood. A personal trainer will be a great aid if learning correct movement patterns and lowering your risk of injury is important to you.