Most coaches and trainers give athletes some leeway in how they do an exercise. For example in the biceps curl, is the arm completely extended at the beginning of the movement or is there is slight flexion in the elbow joint? In most cases the difference is negligible because the end result and its importance in the sport is usually not that important.
There is also some leeway given as to the exact movement, body positioning, and body or barbell pathway or adjustment of the equipment to make the exercise possible. Because of this much attention is usually not given to exact positioning during execution or to the execution itself.
The execution of an exercise is usually described in very general terms without discussing the nuances of the exercise. This is acceptable for general body building type exercises. But there are important exceptions as for example, when it comes to exercises for the lower back and hamstrings. A slight difference in positioning in these exercises can make a major difference.
But positioning in lower back & hamstring strength exercises is extremely important. For example when doing the back raise on the Yessis Glute Ham Back Machine the axis of rotation must be in the waist i.e. in the lumbar spine in order to target the erector spinae muscles through the full range of motion. When doing the glute/ham/gastroc raise the pelvis must be situated over the seat so that the axis of rotation is only in the hip joint.
If you try arching the back with the axis at the hip joint as you rise up, you will have a double axis in the exercise which can injure the lower back. When you try to do hip extension or glute/ham/gastroc raise with the axis at the waist you will not be successful. You will be unable to produce a strong contraction of the hamstrings before you bend the knees. Thus you do not get the same effect.
You should always check to be sure that you are doing the exercise correctly and if the equipment you are using allows you to do it correctly. By doing this you will not only get the results you want, but it will also prevent injury.
For more information on how best to do well over ninety different strength exercises as well as the best equipment to use for these exercises,see Biomechanics and Kinesiology of Exercise.