I learned about the existence of usermode helpers last year by working with coredump helpers (/proc/sys/kernel/core_pattern) and found it quite interesting too. The modes where the kernel spawns a new process on behalf of the users (or as init process) are quite obvious. But this case - where the kernel just executes a helper program in userspace which then immediately terminates - are slightly different. It's a neat design - because it allows for customizing kernel behavior without having to compile a custom kernel.