so i heard about this on npr early this week. the announcer discussed the meaning of the tradition, etc. and described the flails they use. he said it was like "whipping yourself with razor wire". ow. here is a link where you can hear the segment.
npr.org
it was really quite creepy. i cringed at least once listening to it. they have audio of the chant leading up to the act, and the description of the flails they use is... i dunno, not pleasant.
~james