I first read about this phenomenon in Charles Dickens work Pictures From Italy, it was not a sight I could visualize in my head. Maybe it is because I, myself, have never seen or done anything of the sort. Dickens describes it as ridiculous, unpleasant and senseless. However, I do not totally agree with him. The reason being, if you look into the purpose, which is penance you may be able to understand why devout Christians put themselves through this pain.
As I stood at the bottom watching the penitents slowly make their way up the flight of marble stairs, I was in awe. The devout and brave penitents, many middle aged looked in pain as they lifted each knee slowly as the crept up to the top, while still stopping to pray.