I'm not saying that it cancles out every lght..thats just the easy way of saying it.

There is a lot of technical stuff to consider, such as flash type, flash strenght, distance from target, strength and movement of surrounding lights, what types of lights they are.

It's just in places like concerts, where there are alot of moving, high powered lights, you'll find that an ordinary flash will meet those lights half way, and intern cancel out the beams of light, creating a darkened scene.

And I happen to know what i'm on about to, I'm currently completing a BA of Photography.