Prolonged beating of the soles of the feet, or phalanga as it is sometimes called, is definitely torture. It has a rich history. It is extremely painful, primarily because of the abundance of nerve endings in the sole of ones feet. It takes a long time to heal so the punishment effectively gets extended over a long period of time, possibly permanent.

Conveniently enough it leaves almost no evidence.

Edit:A study on the effects of phalanga http://webcache.googleusercontent.com/search?q=cache:_4_CVER...