A Risset Rhythm is a rhythm that continuously increases or decreases in apparent tempo. It's an auditory illusion similar to a Shepard Tone in that it relies on slowly adding new beats that are speeding up and slowly fading out the fastest beats. Because of this the average speed remains constant. All of the rhythms are powers of two in relation to one another which allows them to seamlessly fit together without syncopation giving away the new beats.
It was initially suggested by Jean-Claude Risset in a 1986 paper titled Pitch and rhythm paradoxes: Comments on ‘‘Auditory paradox based on fractal waveform’’ published in the Journal of the Acoustical Society. You can listen to one to an accelerating rhythm here and a decelerating one here. The second video's animation makes it too easy, but with the first video see if you can hear through the illusion by tracking the beat.
IRON NODER XIV: THE RETURN OF THE IRON NODER