Short video/ image overlay?

[theBullringLive]

Wondering if there is a way to have a video clip play over my "main" video when I hit a hot key? I stream motorsports races and would like to be able to have a somewhat transparent -or not, play when racers are crossing the finish line
In case I'm not clear, link to clip
flag overlay

 

[Mikheil]

The only 2 ways I know of to create a transparent video to overlay on a basic video is to save the video as .mov or .webm with alpha channel.

 

[theBullringLive]

But how do you trigger it?