In the video, a tritone separates each pair’s sounds. In each pair, one sound is a mixture of identical notes from different octaves—for example, a combination of two “D” notes, one higher than the other. When the sound is played next to a second note one tritone away (for example, a G-sharp between the two D’s), you may validly interpret the second note as either higher or lower than the first.
Another paradoxical application of tritones is an infinite sound that appears to constantly drop in pitch, though it actually cycles continually. This video plays such a sound for 10 hours.
Listverse also has nine other mind-numbing paradoxes worth pondering. Read about them here.