What does "uninstalled via reboot" mean?

It means that after he is done he reboots his system and his computer will be in exactly the same state as before he installed the giveaway of the day and tested it. A normal reboot will not uninstall anything, but just clear the RAM (random access memory) and some temporary files.

Why is it advisable to "uninstall by reboot"?

If you install a programme on a Windows computer, your system will change. The executive and files it needs to run are generally placed in a folder in Program Files. Possibly some are stored somewhere in the Windows folder. Data can be placed in a normally hidden folder, %AppData%. Also entries can be placed in the registry, a huge database. And also data can be placed in %Personal%, that is the My Documents folder of the user installing the programme.

Running the programme can also result in changes in different places.

If you decide you do not want the programme any longer, you can run its uninstaller directly, or via a utility such as CCleaner. But this does not guarantee the state of your computer will be exactly equal, on a bit-by-bit basis to what it was before you decided to download the giveaway. It can be quite difficult to clean your computer fully and it could be necessary to use several 'cleaners' and even manual edits of the registry, %AppData% and %Personal%.

If you install more than 300 giveaways of the day each year (not counting the reruns within the same year), you can end up with a very messy system. And what's more: if any of them is really buggy, it could do havoc to your system and data in the meantime.

It is much wiser to use modern virtualisation techniques that trick your computer into believing you really did install an application while the installation is only virtual. You can compare it to a dream: it seems every bit real to you (including seeing colours), but when you wake up ("reboot") you realise it was all a fantasy.

"Uninstall by reboot" means you have installed and run an application and when rebooting you get your original system back.

How does he do it?

Many years ago I advised schools to install Reborn cards in computers used by students. The students would try and mess up the system completely and some of them succeeded. That meant the students that used the computer after them could not do their work properly. With a Reborn card the changes in the system were rolled back during reboot. That was a hardware "uninstall by reboot".

Karl uses a software solution, by applying a programme called TimeFreeze. There are various TimeFreeze programmes available. I like the free Toolwiz TimeFreeze. Wondershare used to have a paid programme:

http://blog.wondershare.com/wondershare-time-freeze-free-system-protection-tool/

(the full version used to cost $39, but is not available anymore)

Karl uses a different one: DeepFreeze by Faronics, $45 per year:
DeepFreeze

http://www.faronics.com/en-uk/products/deep-freeze/standard/

(it would be a perfect giveawayoftheday!)
See the manual

Two other "uninstalled via reboot"-techniques I use:
a) SandboxIE - just empty the sandbox and the system is clean again. During installation and usage the changes are written to a part of the hard disk controlled by SandboxIE. The system itself remains 'pristine'.
SandboxIE
b) VirtualBox running a virtual machine. I use a virtual XP Pro that Microsoft gives away to each owner of a Windows 8/8.1 licence. The virtual machine can be kept clean by using a TimeFreezer (double virtualisation) or by stopping the virtual machine and restoring the clean snapshot with a single click:
reboot

Another alternative is Shadow Defender that costs $35 for a lifetime licence. See dottech}http://dottech.org/107370/w... for a comparison of Shadow Defender and SandboxIE.

See also Free Windows Desktop Software Security List - Sandboxing / Virtualization.

The draft of this posting and comments can be read on a forumpage of Giveawayoftheday.