When doing any sort of training it is so important to make sure that you don't just pack a lot of weight on to a bar and do lifts with poor form.
While working in the gym I see a lot of people that try to pack weight on a bar and lift incorrectly, this not only going to hinder their development but they also run a higher risk of injury.
Form of an exercise is about making sure you get the most out of every rep, Prime examples of this is doing a deadlift and keeping your back in a neutral position and driving through the hips to lift, NOT lifting through a curved back. A second example is doing a bicep curl with keeping your elbow locked in to the side and not swinging the whole body as you pull the bar back up.
Drop the weight
When working at a weight that is to heavy for you your form is going to be incorrect and you will be incorporating other muscles to complete you reps. Dropping the weight is going to be better to make sure you isolate the muscle you want work.
Time under tension
Working an exercise in a slow controlled way is going to tares muscle fibres just as well as you would if you lifting a heavy weight, its these tares that will then repair and rebuild and be stronger then before.