The APU (aux power unit, a small turbine used mostly to provide bleed air for main engine start) is in the tail, right there.
Beat me to it. Here is another version of the video without the chemtrail accusations - https://yewtu.be/watch?v=6PvM3VWawuk
And an APU startup without smoke - https://yewtu.be/watch?v=sTKOJqKw4V4
(post is archived)