The relevant paper describes it as "mostly-parallel, mostly-generational". The idea is that it works like traditional tri-color mark and sweep collector, but white objects stay white between partial collections unless they are modified (ie. reside on memory page that was modified and thus caused memory access trap). In essence it seems to me that difference between generational and incremental/parallel mode lies only in what causes white objects to revert to black (ie. if it happens after each sweep (incremental) or only when sweep does not produce enough free memory (generational)).