What does etcher option "trim ext{2,3,4} partitions before writing (raw images only)" actually do?

I could not find any information about option “trim ext{2,3,4} partitions before writing (raw images only)”.
Can somebody explain what it is doing exactly?

Hi,
It’s an option to trim empty space inside those partitions so that the size of the written image might be reduced, with the intent to also potentially speed up the flashing process depending on where said empty space is positioned (e.g. if it’s mostly in one block, it’s gonna be fast - if it’s scattered all around, it doesn’t make a heavy difference)

Ok, thank you for the clarification.
So it does not change the size of the partitions but avoids to write unnecessary data (= empty space in ext{2,3,4} filesystems)