Remember, trying out an early build like this can be risky. That's why we recommend that you don't install the preview on your primary home or business PC. Unexpected PC crashes could damage or even delete your files, so you should back up everything.
After you install Windows Technical Preview, you won’t be able to use the recovery partition on your PC to go back to your previous version of Windows.
"
After registration, when you are on the download-page, the purple download-button links to a small exe-file (4.6 MB).
This seems to be an online-installer that will install W10-TP on your main partition = C-disk !
Which means replacing your existing O/S !!!
Further down at the page:
"Looking for ISO files?
If you're an advanced user and want to install the preview on another partition, another PC, or a virtual machine, we recommend you download the ISO files.