TRIM support is complicated because of some really peculiar ways in which flash-based drives (and thumb drives) work.
The short, quick, mostly technically correct rundown is this:
You can't write to just one byte of a flash memory device. Flash memory cells are grouped together in chunks. If you want to change just one byte on a Flash device, you have to re-write the whole entire chunk.
So in order to change one byte on a flash SSD or thumb drive, the computer has to read the whole chunk into memory, then change the one byte of the chunk, then write the whole chunk back out. That slows down writing significantly. When you add the fact that writing to a flash memory cell is slow to begin with compared to reading it, it takes quite a long time (relatively speaking) to store files on flash gadgets.
TRIM is a way to tell a flash SSD "This whole chunk of space is free sapce. It is not being used for anything. If I tell you to write one byte of data into this chunk, don't bother to read the whole chunk and then write it back out. I don't care if the rest of the chunk is erased when you write this one byte of information, because as far as I'm concerned this whole chunk is free space and I don't care what else is there."
In practice that means that writing is faster. Since it doesn't matter what else is in the chunk, the flash device doesn't have to read the whole chunk, change one byte, and write the whole chunk. It can write just the one byte. Doing that will erase the rest of that chunk, but that doesn't make any difference because that chunk is part of the empty space on the disk.
The down side to this is that normal disk unerase programs like Data Rescue can not reliably undelete files that have been deleted on a flash SSD. If you delete a file, then you write another file, and the other file is part of the same flash chunk as the deleted file (even if it ISN'T part of the same exact bytes), the deleted file will be erased. If oyu use TRIM on an SSD, it can become impossible for file recovery programs to recover accidentally deleted files.
