Bridge Tuplets
A bridge tuplet is a tuplet connecting either two tuplets or one tuplet back to the parent grid. They might look exceedingly complex, but they can easily be optimizations of high-level polyrhythmic ideas.
All bridge tuplets are nested tuplets and can be written as such. However, if you have a tuplet that connects two tuplets with different beat lengths, you would end up with numbers too big to think about as tuplets and the whole thing would be a disaster. With bridge tuplet notation, the numbers are much friendlier and the rhythms are a lot more conceivable.
It’s very important to mention that bridge tuplet notation works because tuplets represent notes at different speeds. In the Bridge Tuplet Visualization on the right, the little 6:4 simply represents 6 sextuplets against 4 eighth notes, because the big septuplet and sextuplet just change the speed of the eighth note.
Fizz musician Israel Strom explains this concept in his YouTube video 6 Levels of Polyrhythms Explained