How does Pregen actually work?

so i want to pregen 1000 Chunks but when i leave the server the CPU usage drops from 90% to 5-10% and i doubt that its actually effectively pregenerating chunks.. when i am ingame it is sending me chunks and potentially generating them at the same time.. is this a bug or am i doing something wrong?

Solution

pregenning normally only works when someone is online yes, because there needs to be someone to request chunks from the server