That yellow flash is called a film burn, like MatRich said - its a throw back to the film cameras of old, where it was naturally occuring when you started filming. Now it is a digital effect tossed in by editors. Most people use it as a transition (like in that video), but you don't want to abuse it; even there you could tell it was out of place. They get old really fast but if used CORRECTLY can be really sick.
If you want some free ones to toss into your vids, try artbeats.com and look around. I downloaded some stuff there and all I do is take down the opacity and overlay it on my timeline.