A field guide
What happens when two different rhythms share the same bar — set against each other, they never quite line up, except where they must.
Start with a single rhythm. Every meter groups its beats differently, and that grouping is what gives each one its personality. The rule is always the same: the first beat of every group gets the accent — so once you can see the grouping, the accents stop feeling random. Pick one, hit play, and listen for where they fall.
Now play two of those groupings at once. A "5:4 polyrhythm" means 5 evenly-spaced beats and 4 evenly-spaced beats sharing the exact same span of time — so one pattern is audibly busier than the other, and they only shake hands at the start and end of the bar.