The weary jaded feeling of "letdown" that one usually experiences after a much-anticipated event (such as a talent-concert or other much-hyped cultural bash, dinner-date, "important" business meeting, etc.) which turns out to be significantly less satisfying or worthwhile than had been expected or hoped for. So, like the "Doppler Effect" that is heard as a noisy vehicle zooms by (the sound of the jalopy's engine is higher as it bears down on your position and then drops drastically in pitch as it rushes headlong into the distance), one's mood and emotions suffer a similar "roller-coaster" or "sugar-crash" journey... you are all keyed up and excited beforehand with your heartbeat going a-mile-a-minute as you eagerly anticipate the looked-forward-to event, and then afterwards you feel disappointed and exhausted, with your whole being slowed into a state of draggy disillusionment.
Cool dude #1: Zheesh, man --- THAT garage band really sucked... dunno when I've ever heard worse performing!
Cool dude #2: Ya, fer shure, buddy --- major Droppler Effect from THAT one!