A polyrhythm is two or more even pulses played over the same stretch of time: three evenly spaced notes against four, for example. They show up everywhere from West African drumming and Indian rhythm to fingerstyle guitar, where the thumb keeps one pulse while the fingers play another, and to the triplet-over-straight feel of blues and rock solos.
Pick a preset or set each layer from 1 to 16 beats per cycle, and add a third layer if you like. Every layer has its own sound (clave, cowbell, hi-hat, tabla, a guitar tick and more), its own volume and its own ring of lights, and a hand sweeps round so you can see where each note falls. Below, a grid cuts the cycle into equal steps and shows exactly where the layers meet, the best way to understand why 3 against 4 feels the way it does.
Set the tempo for either layer, tap it in, and add a one-cycle count-in. To practise, the drop modes silence one layer every few cycles so you play it yourself, and swap alternates which layer goes quiet. A soft grid pulse helps you place the notes at first, and a speed trainer raises the tempo cycle by cycle.
Timing comes from the same audio clock as our metronome: each cycle is divided into the least common multiple of the layers and every note is scheduled ahead, so the layers never drift apart however long you play.