What is this transition?

The effect you see is an light leak overlay. Just google "light leak transition" and you will find thousands of them (some free, most paid). It may take a lot of searching to find this exact one, or you can just pick a similar one.

Mist of these light leak transitions are just video files that you put on top of your clips on the timeline, centered on the cut between two clips. Then you select the light leak clip and in the Inspector > Video tab under Compositing you set the blend Mode to "Overlay" to make the effect clip blend with the clips underneath.

- Ronny

I cannot find the blue flash. I have thousands of light leaks and film burns. I think this was done within a programme

If you want to make it manually you'll have to go into motion, make a new FCPX transition, create a shape and then start messing with scale keyframes, shape keyframes, glow, color, and blur filters to get the look you want.

at 00:42 no light leaks involved, imho ....
within the first 4 frames, clip#1 gets blurred and highly over-exposured
clip#2 comes in as a clean cut, no fade, blurred/overexposured and then within 4 frames back to normal




should be manageable to reenact that in Motion5 ......
or, 'manually' in FCPX ... if the color board allows values so high off ......

addendum
works like charme:
animated Gauss blurr, within 4 frames to 50%
drag a copy upon it, exposure 'correction' +70%, animate opacity from 0-100% within 4 frames ....



ok, my Bad Boy is a pumpkin ... btw: Happy Halloween! :woohoo: