Form is important, but there is a time and place for using a little momentum and loosening up a bit to go heavier.
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Walk into any weight room and you'll typically see people at one end of the technique spectrum or the other. Some lifters are very strict, sometimes too strict as they never use appreciable loads due to their commitment to robotic technique. On the other hand, you find lifters who'd see better progress if they significantly reduced the weight and stopped relying on momentum and generous spotters to complete each rep.
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There's certainly some wiggle room in terms of acceptable and efficient form. It's even been shown that slight momentum can increase torque requirements and muscle activation. Your form will also breakdown a bit if/when you test your max at strategic points in the year. A true one-rep max never looks textbook-perfect. If it does, you could've gone heavier.
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However, most of the time you do need to be very strict with your exercise form, and you need to learn the right type of form for your body on various lifts. This is especially important for bigger exercises like squats and deadlifts where the risk of injury is inherently higher than, say, dumbbell curls. Failure to pay attention to technique will result in pain and injury, which will stop progress in its tracks.