Hehehe.... well basically this is what I did...
I set my camera's shutter speed at 4sec with a rear/second curtain flash. Set the camera on a tripod.
My brother (the subject with the drinking glass) held the glass as if he was pouring something on his hand. By the way there was no liquid involved in this photo.
When I pressed the shutter... my girlfriend, using a small flash light, started inside the drinking glass and moved the flashlight towards my brother's hands. My gf turned off the flashlight when she touched my brother's hand.
The 4sec shutter speed allowed the camera to capture the light and the light's movement. The rear/second curtain flash allowed the camera to capture the rest of the picture (my brother, the couch, the glass, the wall, etc...).
Without the second curtain flash all you would see is a dark picture with a bright "tube" of light.
I am not really good with explaining things but I hope you understand what I was trying to say. If you have any other questions please let me know or check out this tutorial...
http://digital-photography-school.co...ow-sync-flash/