I've been told it's called a "hera", you know, the plastic bit that keeps the koshita next to your back.
...so, how do you (personally) prevent the buggers from ripping off?
Other than becoming good at aikido that is
I am tempted to go commando i.e. sans hera, but being a kendo guy this seems a bit of a sacrilige! And it might make my koshita go all floppy and dangly... most undignified!
When my one ripped off, for the second time, i tucked in the himo at the back under the waist loop i had made with the front himo which seemed to do a trick but the old mighty hara (AKA beer gut) prevents me from giving it the once over to see if it looks nice and neat, again probably the kendoka in me coming out.
Peace and love