As far back as at VMworld 2014, VMware announced VMware Project Fargo technology, also broadly known as VMFork. It allows to make a working copy of a running virtual machine on VMware vSphere platform very fast.
The VMFork technology involves on-the-fly creation of virtual machine clone (VMX-file and process in memory), which uses the same memory (Shared memory) that the parent VM does. At the same time, the child VM cannot write to the shared memory and uses the allocated memory to write its own data. With disks, it is just the same: with the Copy-on-write technology, the changes of the parent VM base disk are written to the child VM delta disk: