The Leg Raises in yoga are generally intended to prepare the body for the asanas to follow.
The muscles which are specifically straightened and toned by yoga leg raises, are the abdominal and lower back muscles, together with the muscles of the legs. If you do not have strong muscles at first, you will probably find this routine difficult to perform. But, after a while, the moves will become easier to do with time and practice.
If you are not at the best physical level, you are probably going to have a problem with arching your lower back, or using your shoulders to help lift your legs at first. You will need to get past this because, in order to maximize the effects of the exercise, it is very important to have the full length of your body resting on the floor, with your shoulders and your back are relaxed.
You start yoga leg raises lying on your back on the floor with your legs together and your palms down on the sides of the body.
You can either perform leg raises by raising one leg at a time, while the other one remains on the floor, or, by raising both of them, at the same time. If you choose to work only with one leg, you can start by pushing down with your hands, to help the leg lifting.
To achieve best results, keep your knees as straight as possible, and try to flatten your back to the floor as closely as possible.
As with all other yoga asanas and poses, when performing this routine remember your yoga breathing, as deep breathing can contribute to faster and more satisfying results.