These asanas rely on gravity to bring the trunk of the body forward. In many the hands are raised above the head such as in Pada Hastasana (hand to foot pose). Forward bending asanas loosen up the back, gently stretching the back muscles and separating the vertebrae whilst the compression in the abdominal area provide a gentle massage for the digestive system. An exhaling breath is used as you bend forward activating relaxation and there is a general sense of letting go.. “surrendering to the moment”. When bending forward it is important to lead with the chest, to use the abdominal muscles to support the lumbar spine and to bend from the hips not from the waist.
Of course care must be taken with forward bends to ensure that you do not force the back to bend further than its present flexiblity will allow. Forward bends also put some stress on the lumbar spine particularly as you pass through the 20 degrees so people with lower back problems especially disc problems need to seek advice from a qualified yoga teacher and may need to check with their doctor. Often the practices can be performed in a modified way with shorter levers and for those people with high blood pressure you can ensure that the head does not move below the heart.
Forward bends such as Shashankasana (pose of the child) can be very soothing and are often a good way to release anger.
Forward bending poses should be followed by a backward bending pose to balance the practices out. A good one especially for beginners is also a relaxation pose known as the Crocodile pose (Makarasana)
Some useful links for yoga and the management of back pain are http://www.yogamag.net/archives/2000/cmay00/back1.shtml