I've seen a photo with a nice light streaks illusion, that I believe was captured in the camera itself rather than added in post-processing. Can anyone explain how one can replicate it?
It looks to be a long exposure in a dark room using a flash that illuminates most of the field of view for only an instant. You can see the strobe flash being held in the lower right corner of the image.
The streaks of light are caused by camera movement and the dim bulbs in and around the art on the wall in the back of the room. It seems that in this case the camera was not only moved in the x and y axis, but also rotated around and moved in or out on the z axis. It appears the lens may have also been zoomed during exposure.
For several other questions that show the various effects that can be achieved by shooting points of light in a dark scene using camera/lens movements during longer exposure times, please see:
How do I get started with 'painting with light' photography?
What is the photography term for blurred / dragged lights?
How do I choose a lens for a "glow in the dark" indoor/lowlight event?
How is the circular motion blur created in this Instagram image?
Sometimes such effects are not desired:
Strange light when shooting long exposure
Why in a photo light lines of street lamps? How to remove them?
One possible way to achieve the look is with multiple exposures. One image is taken with near zero light in the room. With the shutter set to Bulb, the photographer opens the shutter and keeps it locked with a cable release. Then with a flashlight, the photographer "paints" in all of the streaks.
The 2nd exposure is then made with the flashlight pointed at the subject. Of course, this could have been done in reverse order, too.
Yet another way would be to make a composite of two different images.