The VSAN Team has released a new whitepaper (Tech Note) on how the new snapshot technologies work in VSAN 6.0. Below is a link to the document.
Virtual SAN 6.0 introduces a new on-disk format that includes VirstoFS technology. This always-sparse filesystem provides the basis for a new snapshot format, also introduced with Virtual SAN 6.0, called vsanSparse. Through the use of the underlying sparseness of the filesystem and a new, in-memory metadata cache for lookups, vsanSparse offers greatly improved performance when compared to previous virtual machine snapshot implementations.