While there are many philosophies and methods out there today. This is what I have found to be most effective over 3 decades of application.
Importance of breathing when lifting weights. It ensures that the blood circulating to your working muscles is oxygenated and that waste products are removed. Holding your breath during weight lifting can lead to dangerous increases in blood pressure, which can lead to injury. Use your breath for you, not against you. i.e. When you do a bicep curl and bring up the dumbbell, breathe out.
When you lift weights, you build lean muscle tissue which is more metabolically active than fat. When you increase your muscle, you also increase metabolism which means you're burning more calories throughout the day. Regular strength training is just as important as cardio exercise for losing fat and getting fit. Utilizing a target weight for your desired number of repetitions leads to optimum development and performance.
One of the most important reasons to maintain proper form during weight lifting exercises is to prevent injury. When we lift a lot of heavy weight, this can cause the body to become misaligned that can place your tendons, muscles and joints in positions that can potentially cause strains or tears.
The HIT method that I was trained with personally by Arthur Jones, founder Nautilus, and the HIT method of weightlifting, is to lift the weights steadily under constant tension for 3x3 seconds, then release them slowly for three seconds. This varies from the usual method of lifting for one to two seconds.
Concentric movement is when the muscle shortens while producing force (contracting the muscle). This happens when you are raising the weight during a biceps curl. Do this slowly but deliberately.
Eccentric movement is when the muscle lengthens while producing force. For example, when you're lowering the weight back down during a biceps curl. Do this slowly but deliberately.
You can sit on the floor, sit on a stool, kneel or stand.
While all of these positions achieve the same result, the less leverage and swinging that happens the better the result and isolation.