Can someone explain this a bit more to me?

[quinlanphotographic]

An interesting effect when photographing dim areas with background lighting.. nightclubs for example:
- Set your camera for flash 2nd curtain
- Set your aperture (letsay f/5.6) and meter to determine shutter. The shutter setting will most likely be very slow for handheld. Thats ok... Increase the shutter by a stop or two to intentionally underexpose.
- Set the flash to properly expose the subject at that aperture

The flash will freeze and expose main subject. Motion and background blur will also be captured.

 

[Dex_star]

ur background will be underexposed since you set the shutter lower than metered... but the flash will light ur object and expose it perfectly (hopefully)

Its an effect that helps ur main object pop more

... idk if that was what u were asking for?

 

[reg]

Dragging the shutter. What more do you need explained?