A finely judged bit of flying, or more automated magic. The drop in air pressure above the wing is causing water vapor to condense into water droplets. As the wing moves out from under the droplets, the pressure rises and the droplets evaporate back into vapor.

To emphasize the point, the edge of the flap divides low pressure above and high pressure below. The high pressure escapes from below and that produces, on this side, a clockwise rotation, viewed from behind. The vortex created by that rotation accelerates and the same low pressure condensation happens. Hence the slight gap between the flap edge and the start of the condensation "streamer".