That clip was from the only indoor practice the Chiefs have held this year at Missouri Western State University. Gaines injured his knee during that practice and it's possible the injury happened on this play.
Nice move by Thomas to shake Gaines here, but it's nothing like the open-field juke that Jamaal Charles put on Sanders Commings during Chiefs training camp last year. I saw that one. Charles screwed Commings into the ground. Commings suffered a broken leg and a badly sprained ankle on the play and was done for the season.
No video of that play has been posted that I know of. If it's out there, I'd love to see it.