When you are training calves you want to avoid movement that will transfer the load onto your Achilles tendon from your calf muscles. The best way to do this is to keep constant tension on the calf muscle through the entire set by keeping your calf flexed as hard as possible through the complete arc of movement. This will be hard to do the first few times you try as you need to teach your muscles to stay contracted while they move through the movement of the exercise. It's easiest to learn this by starting with seated calf raises. Position yourself in he calf machine and remove the safety placing the load on your calves. Reach down and feel your calves as you apply tension to your calf muscles by flexing them. Slowly raise the weight keeping your calves flexed. When you reach the top of the movement flex as hard as you can for a 3 count then slowly start to lower the weight while keeping the muscle tight (this is the tricky part). You will really need to go slow and concentrate on keeping your calf muscles flexed. By holding your hand on your calves you will be able to tell if the tension goes away or remains through the whole movement.
When you stretch your calf muscle as far as it will stretch under a load, it becomes hard to keep tension on the muscle and if you can't keep tension on the muscle then it can't contract as hard as it possibly can, stimulating growth while you lift the weight. If you over stretch on the bottom you will take the workload off the calf muscle and distribute it elsewhere which will result in a lot of work with very little calf muscle growth. You only need to lower the weight a little but not all the way to get enough work done to grow.
So often you will see a bodybuilder bouncing at the top of the movement trying to get a peak contraction. Since the Achilles tendon and not the calf muscle is being used to lift the weight into position there is very little benefit from getting the extra height. At very best you perform an isometric contraction at the very top of the movement after the weight has been bounced into place and isometric contractions are not effective for building size. As you perform your calf raise with tension on the calf muscle through the whole movement you will reach a point at the top of the movement where you cannot go any higher without bouncing. Don't Bounce! When you are at the top of the movement just hold the contraction for a 3 count then start back down again. Getting as high as you can on your tip toes by bouncing is completely ineffective for building calf size and just because your muscles are burning doesn't mean they are being stimulated to grow, it just means you have a high amount of lactic acid in your muscles from metabolizing glucose. You can get a burn and not have stimulated your muscle effectively enough to grow.
By performing each calf exercise with a controlled, steady pace, it is much easier to keep constant tension on the calf muscle and you need that tension to be able to contract hard enough to stimulate growth. Think of your car for a minute. It will go 60 miles an hour but it takes a few seconds to get there. Your muscles will contract very hard but that is not instantaneous. If you keep tension on the muscle all the time it is much easier for that muscle group to contract near 100% of it's capability much quicker. The harder you can contract through a movement the more that muscle group will ultimately grow. Theoretically, the purpose of weight is to force the muscle to contract harder by adding resistance but in the real world the load is not always handled by the muscle alone. Keeping tension on the muscle will help focus as much muscle building tension on those stubborn calf muscles as possible.