What is WinPE:
WinPE is a cut down version of Windows 7. It can boot from a CD/USB and sit in memory so that you can work on the disks without causing a conflict.
The windows deployment application
sits on WinPE so it can be used to image and manipulate disks.
Boot the host computer via the ISO boot disk (which is actually WinPE with deployment software on top).
The WinPE will load and then the deployment software will be loaded on top.
It can be used as a deployment tool at this stage.
The user can drop out of the deployment software by clicking 'cancel' and work on the ensuring DOS screen.
This is the point that usual often make a mapped Z drive.
You can run 'DiskPart' from here too (for tech people)