This kovarex design uses constant combinator signals to set inserter filters, limiting the amount of U-235 buffered in each centrifuge. This is possible using an integer overflow trick: When the centrifuge's items exceed a threshold, the value in the centrifuge + the value in the combinator overflow past 2^31-1 to a negative value, which is then excluded from inserter filters.
The red / blue belt versions have slightly different settings, and the legendary version is adapted for stack inserters.