Guild Wars 2

I was watching this video on youtube https://www.youtube.com/watch?v=SqmcIFekewE

at 0:04-0:07 he was able to turn around his camera in a similar manner as to checking your “rear view mirror” while driving without using the “about-face” hotkey

How do you manipulate the camera without altering your walking direction?

It seems to be quick helpful and fluid in terms or pvp.

please tell me how you did it.

Thats the “Look Behind” keybind. To set that you need to go into
(Options → Control Options → Camera → Look Behind) and assign a key.