This effect automatically strobes between 2 source clips with a variable speed.
It has been setup to work with all SD, HD, 4K & 5K resolutions at all available frame rates from 23.98 to 60fps in FCPX.
It also has a PDF file inside the folder. This folder, as downloaded from the link below, should be put inside a "Custom" named folder in your "Effects" folder inside your "Motion Templates" folder in your "Movies" directory.
Unzip the file by double clicking and place the folder inside your Users/Movies/Motion Templates/Effects folder. Then, in FCPX, locate it in the Effects browser and drag it onto a clip in your timeline. Select the clip you dragged it onto and in the video inspector you will find some adjustable parameters.