Explanation Shared options (Hash mode and buffer size)

I have just bought and installed Teracopy and it looks promising. 

But during my first 'project' I discovered some (new) setting 'Shared options' from which I cannot find an explanation in the help files / manual. Especially regarding the buffer size.

What does it do and why is 1MB buffer size the default? If I increase the buffer size does it effect the speed in a positive way without loosing quality (duh ;-)) ? 

Are there scenarios for setting the buffer size? For example: if you leave the computer unattended while copying you can change the buffer size to a higher number or a lower number if you working on the computer while copying??

Thanks in advance.