We use it to service the clutch once it is removed from the machine. It holds the clutch where you can work on it in a vice and disassemble to clean and lube or adjust spring position. I will try to get a picture of it on here, it works very slick.
The boss that holds the idler shaft is ovaled? Wow, have never seen that but it answers the question of why it pops out of gear... good luck.
//RR