Think of the strings as columns of soldiers in the Bernoulli Brigade. They seem to have had a jolly time last night. (Parenthetically, "Taps," the mournful bugle tune played to summon soldiers to their barracks for the night, gets its name from the taps of beer barrels in the local pubs.) Columns of molecular soldiers are thrown into violent swirls by the ends of the flaps, each of which generates its own private tip vortex (whose core is visible, if you carefully pick your seat in a landing jet on a moist day, as a trembling gray rope). The flow over the middle of the flap veers inward above the wing and outward below it, producing a scissors-like shearing at the trailing edge. The low pressure generated by the flap distorts the ranks of streamlines on the outer panel, pulling them inward, while the vortex at the tip of the wing is quite weak, indicating that not much lift is being generated out there.