Bridge Tuplet
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
Note that notation softwares and other websites are much more friendly to nested tuplet notation, so you'll need to convert your bridge tuplet to be notated as a nested tuplet in some cases. Fizz musician Jonathan Berroa breaks down the conversion in his Youtube video Bridge Tuplet to Nested Tuplet Tutorial