Nikon latest DSLR cameras have very good feature to lock different exposure settings and other parameters like Autofocus, Focus point area etc . Meaning you can lock auto exposure, auto focus, flash value as per your need. Last time we talked about Auto Exposure Lock (AE-L) and how to use this feature. Now lets talk about Flash Value lock feature. The basic idea is common between these two features. Auto Exposure Lock feature is used to lock exposure value and Flash Value Lock is used to lock flash power for proper exposure of subject and background. This feature comes very handy when you are shooting in dark situation and your subject is not centered in your frame. Usually camera meters centered subject of your frame to calculate amount of flash light to be needed to properly expose your subject. If your subject is not centered, flash value given by camera may not match for proper exposure resulting overexposed or underexposed subjects with background. You can program your camera function button to activate Flash Value Lock (FV Lock) feature or you can even use AE-L/AF-L button to program it. When you program these buttons for FV Lock, next thing you have to do is point your flash towards your subject and push the FV Lock button (which you just programmed). This will fire pre-flashes briefly and lock the flash power to properly expose your subject no matter what is the light condition in the background. Once you lock the flash power, you can recompose your photograph as you want. Now when you are ready to take a shot and press shutter release button, flash will be fired with power you previously locked into. This will give proper exposure to your subject ignoring ambient light or dark situation in the background.
In this photograph, there is a mix background for the main subject. There is a dark sofa and bright Teddy Bear. She is wearing dark color dress and sitting on the sofa with bright object. This kind of situation can confuse camera light meter to fire the flash with proper exposure settings. That is why I locked the flash power to properly expose subject and re-framed it and shot it again.
Normally you should use FV lock button to meter a subject that will be off-center and the background is dark.