A polyrhythm is two different pulses happening simultaneously. The most fundamental one — and the gateway to all others — is 3 over 2: three evenly-spaced notes occupying the same span of time as two evenly-spaced notes. It feels like one pulse arguing with another, and resolves only when they finally meet.
This lesson installs 3:2 as a groove rather than just an exercise. The bass drum holds the 4-pulse (the steady ground); the snare plays quarter-note triplets across it (the 6-pulse, which is a 3 against the 2-pulse you can extract from beats 1 and 3). They only line up on beat 1.
Exercises
4A — Quarter-Triplet Polyrhythm Groove
Snare plays the 3-pulse, bass drum holds the 4-pulse. They only line up on beat 1. Feel the 3, count the 4. Mantra: say 'hot — cup — of — tea' with the 4-pulse, while feeling 3 evenly-spaced taps.