A good example of how to use a differencing disk is to install an OS on a virtual disk and then create a differencing virtual disk and all changes you make to the system will be written to the differencing disk.
To create a differencing disk you have to follow the same procedure as you create a new virtual disk but on the Choose Disk Type, you need to choose Differencing.
Then on Configure Disk, you have to choose the disk that will be the parent disk for the differencing disk.
With PowerShell you create it like this:
new--vhd c:\diffVHD.vhdx -sizebytes 2gb -differencing -ParentPath C:\newvhd.vhdx
I have noticed that the differencing disk must be the same size as the parent disk.