Almost all of the yoga poses (asanas) work to increase your spinal flexibility in a forward backward position, but there are only a few which can give you better lateral flexibility.
The half spinal twist, or ardha-matsyendrasana, does exactly that, and it completes the series of poses and exercises that develop mobility and overall fitness. Apart from increasing flexibility this pose also improves the response of the spinal nerves and it helps massage internal organs.
The pose should be carried out while keeping the spine erect and making a sideways circular motion. The shoulders should be kept level as the twist is performed. Balanced breathing is also vital while performing this pose and you should increase the level of twist with each exhalation.
To begin the position sit on your knees while keeping the legs together and resting the buttocks on the sole of your feet. Move your upper body to the right of your feet and lift one leg and place it over the other one. The foot goes right next to the knee of the other leg. The body slowly twists and your hands should follow the position of the torso. During this motion the spine should remain straight for best efficiency.
The next step is to bring your right arm (if the twist is performed towards the left side) on the left knee. Hold the left foot in your hand as your left hand rests behind you. The head should also be involved in the twist, while looking over the left shoulder.