From the description it's probably a Bacon cycle collector. The basic idea is that it checks to see whether reference counts for all objects in a subgraph of the heap are fully accounted for by other objects in that subgraph. If so, then it's a cycle, and you can delete one of the edges to destroy the cycle. Otherwise, one of the references must be coming from "outside" (typically, the stack) and so the objects cannot be safely destroyed. It's a neat algorithm because you don't have to write a stack scanner, which is one of the most annoying parts of a tracing GC to write.