Put the bass drum on all 4 beats, then lay 6 evenly spaced snare strokes across the same bar. Nothing in that bar is fast and nothing in it is a fill, but the floor moves. Two parts are now running at different rates through the same span of time, and neither is a subdivision of the other. That last part is what makes it a polyrhythm instead of a busier groove. 6 against 4 reduces to 3 over 2, which is 3 evenly spaced notes in the time of 2, and every harder ratio gets measured against it.
Exercise 1 writes that ratio as a one-bar loop rather than a drill. A polyrhythm you can only execute cold from a dead stop is a party trick; one that loops is something you can drop into a tune and leave running while the band plays over it. Notice what is missing from the page, too. No hi-hat, no ride, no ghost notes, just two voices carrying the two pulses, so nothing covers for a stroke that lands in the wrong place.
Where the kick and the snare actually meet
Both parts sit on one hidden grid: 12 even partials per bar, which is the 8th-note triplet grid counted 1-trip-let 2-trip-let 3-trip-let 4-trip-let. The snare takes every second partial and the kick takes every third, and that fixes the shape of the bar. They strike together on beat 1. The kick on beat 2 lands exactly halfway between two snare strokes. They strike together again on beat 3, and the kick on beat 4 sits halfway between two more. So the figure closes every 2 beats, not every 4, and the bar is a single 2-beat cycle played twice. That beat 3 unison is the checkpoint for everything that follows.
Counting will put the strokes in the right places. Hearing the composite is what makes them groove. Add both voices together across the first half of the bar and you get four attacks in a shape most people already know: hot-cup-of-tea. Hot is the unison on beat 1, cup is a snare stroke, of is the kick on beat 2, tea is a snare stroke, and the next hot is the unison on beat 3. Say the phrase out loud twice through a bar before you play anything. Once it is in your mouth, your limbs are only handing out syllables.
Building it against a quarter-note click
Set the metronome to quarter notes at ♩ = 75 and let it double the bass drum. A click on the triplets would hand you the grid and do the hard part for you. Play 4 bars of kick alone against it, then bring the snare in on scaffolding: those 6 strokes land on 1, the let of 1, the trip of 2, 3, the let of 3, and the trip of 4. Nothing in the bar is accented, so keep all 6 snare strokes at one volume and let the kick sit at a weight that matches, otherwise one pulse becomes the melody and the other becomes background. Then take the scaffolding away, because the cue that survives at tempo is shorter: feel the 3, count the 4.
This fails in a predictable direction, and the exercise tells on you when it does. If your 4th snare stroke does not land flush with the kick on beat 3, the 3 drifted somewhere in the first half of the bar. The usual culprit is stroke 2, which belongs two-thirds of the way through beat 1, on the let, and badly wants to arrive early on the &. Every stroke after it inherits that error, so you either reach beat 3 ahead of the kick or feel the gap opening and stab at beat 3 to catch up. The other giveaway is that the bar suddenly feels easy, which usually means the snare stopped floating and snapped onto straight 8ths, putting strokes 2 and 3 on the & of 1 and the & of 2. When that happens, drop the snare, play a bar of kick with the click while you say hot-cup-of-tea twice, and re-enter on beat 1. Loop at ♩ = 75 until the beat 3 unison arrives without you aiming for it, then work down to ♩ = 70, where every gap is audible, and up to ♩ = 85.