The timing parameters are in % of length of clip which means that if you have a 10 second clip and have Start in at 0 and you set the End In to 10 (%), you have a one second transition from the corner to full screen — which is 10% of 10 seconds. If you keep the same settings and you shorten the clip to 5 seconds, the transition will be 1/2 second; lengthen to 20 seconds the transition will be 2 seconds. [For a 20 second clip with a 1 second transition, you would need to set End In to 5 more than Start In... okay? Most of the time you can simply "eyeball" the settings — it usually doesn't have to be that exact — whatever looks good.]
I set the default settings for a 10 second clip with 1 second animations at each end.
When you set all the timing parameters to 0, the clip stays "stuck" at its corner throughout the duration.
You can set the Start In/End In to 0 and the clip will start full screen and animate to its designated corner according to the Start Out/End Out parameters... and vice versa — you can set a clip to start in its corner then animate to full screen based on the timing. Again, all timing depends on how long the clip is that the effect is added to and that's why you start out "assigning" corners and setting the animations to zero, then clipping out what you need to animate and moving it to the "top" of the stack (otherwise the full animation is obscured by the clips stacked over the clip.)
Basically all you have to do is set up the corner videos with all timings set to 0, then whenever you need to expand one fullscreen, make you blades for the section and reset the timing (if you go all the way to the right edge of a parameter setting you will see a backward arrow —similar to ⟲— that you can click to reset the parameter to its default setting.) Then move that segment up to the top of the clip stack. If the clips are longer or shorter than 10 seconds, you can retime the animations with the End In/Start Out settings.
BTW, setting all the animation parameters to 100 will have the same effect as setting them all to 0.
The Easing parameter can be set to "linear" with a setting of 0, otherwise it is Ease In/Ease Out at 100%. Anything in between is a "mix" of the two.
It occurs to me that if you use different aspect ratio clips, for example vertical and horizontal in your project, make the clips that don't match the project a compound clip, then add the Corner to Fullscreen effect to the Compound Clip. Corner to Fullscreen works with any size clip but will animate to whatever aspect dimension the clip is (if that makes sense). In other words, if you apply it to a 9:16 vertical clip in a 16:9 project, the corners will be maintained within that 9:16 vertical space and not the 16:9 project space. "Wrapping" the clip in a compound will make it function according to the project size.
