Hey,

I see many of those guys. I think it's because a keirin frame doesn't have front-brake mount and front-brake mounts are in demand and not usually available. I also think many people are taught to slam rear-brakes in case of emergencies. (THIS IS WRONG; THIS IS DANGEROUS. http://sheldonbrown.com/brakturn.html )

I think this rear brake option is poor. For one thing, you aren't going to get good braking. And, you are attaching brake as the secondary "independent" braking system in case your rear tire fails from flat or skidding too much. (Am I wrong?)

Then again, there are many people who are extremely skilled at skid-stopping. Many of these people may have some brake system to show police officers when they get stopped and stuff. And, let me tell you front-brake attachment is big and ugly.

T