The process works something like this. Imagine you have a bin with 100 sweet, juicy, red Jonathan apples. One night someone removes 50 of those apples and replaces them with 51 tart, green, Granny Smith apples. You wake up and get a report of your inventory that shows you now have 101 apples. The extra apple might be a normal change in inventory, or simply an error in counting, so no problem, right?
The trouble is that you no longer have 100 Jonathan apples plus one odd one. Instead, half your inventory is different. The composition of your bin has changed in a way that wouldn't be noticeable from a simple count of apples.
This is what happens with churn in voter rolls. By adding and removing roughly equal numbers of registrations, the total remains almost constant, with any changes in totals within a normal, expected, range. In reality, though, legitimate voters are being replaced with questionable registrations that can be manipulated to serve the goals of election fraud.
But how does this affect elections? In the eight states with universal vote by mail, ballots will be sent to every active listed voter in the rolls. This includes the registrations that were added during churn. In most other states, there is widespread use of absentee ballots.
Unlike ballots sent to legitimate voters, ballots sent to improper added voters will be sent to addresses under control of the fraudsters. These ballots can be collected, filled out as desired, and returned by mail without any verification of voter identity, or even existence.