SkunkyArt | JeanFan — The Source

Turn off all lights such that the room/studio is for the most part completely dark.

Pose the model in the desired position (including hands).

Set your camera to Manual Mode at some slow-ish shutter speed (I like to fix my aperture too to control the DOF).

Sync the external flash to trigger in the beginning of the exposure to capture the model prior to the introduction of the blue light.

Now trigger the shutter/take the photo (I usually like to use IR remotes to do this). This is when the flash should also go off.

Introduce the blue light after the light from the flash dissipates and wave it around in the model's hands for the rest of the long exposure (try not to bump the model's hands too much though it's hard not to).

Note I: The cords of the blue light and the hand waving the blue light do not end up in the photo due to the lack of light hitting them. If light does not hit an object, it cannot register on the camera sensor (or your eyes I guess).

Note II: The model appears more-or-less perfectly still because of the initial flash. The model still held relatively still for the remaining exposure since the blue light was still hitting her hands and registering on the camera sensor as seen by the blue reflection in her hands.

Note III: The blue light appears spherical or ball-like because the blue light was waved around in that configuration for the length of the long exposure. This is probably the part of the shoot that took the most time to get right.