If I remember correctly, that's an additional interface for usermode initiated, cooperative scheduling of kernel threads (basically a variant of pthread_yield that allows specifying which thread is going to get the timeslice we are relinquishing).
It still requites a syscall and the entities being scheduled are proper kernel level threads.
It still requites a syscall and the entities being scheduled are proper kernel level threads.
So not really fibers. Pretty cool though.
Edit: autocorrect