which light 'flashes' when you take a portrait.?

If you are using incandescent light, none of them "flash"

If you were to use your on camera flash, it would shift the image toward blue since the colour temperature of flash is much higher than incandescent.

If you are using a studio lighting setup with studio strobes, then the "modeling" lamps are constantly on, but the flash tubes in the heads are what "flash" when you make the exposure.
 
My fill light and key light and hair light are monolights, so they have a modeling light on all the time and flash when triggered. My background lights are just simple slave strobes. When I push the shutter down, its a big flash in the whole room. They all flash. I don't have any constant lighting that I use for lighting a shot. I have a small flood bulb that I have on a tripod so that I have enough light to comfortably focus the camera (Olympus cameras are not good in low light) and that's the only constant light source I use. The strobes are overpowering enough that they make that little bit of light invisible in the photo.

So I guess you could say they all flash. One triggers and the others are all slaves. Nothing that shows up in the photo is a constant light.
 
What sort of camera do you have?

It depends on how much light is needed to light up the subject. The fill flash will be used if you are taking a portrait photo of someone and the sun is directly behind them.
 
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