How to do this transition?

[jeb_b_vt]

Is it a mask of some sort? I use FCPX and have been wondering how to do it everyday since i saw it. At 2:35. Any video tutorials would be great! Thanks, +K for helpful responses

Park City Night Session from Toy Soldier Productions on Vimeo.

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[gravel]

they masked out parts of each on keyframes and then layered the two with a keyframe movement that slides along the clips as a transition. it's After Effects level stuff.