Jumpscare Phantom BB is Phantom BB as he appears in the last frame of his jumpscare in FNaF 3 and Ultimate Custom Night.
Physical Appearance
He appears the exact same as Phantom BB, except he is inverted.
Mechanic
When he appears on your monitor, switch the camera or close the monitor. If he's kept on the monitor for too long, he will invert the colors of your view for a few seconds.