summarized another way, you can't write zeros to flash memory, you can only write 1's. If you want zeros you have to zero an entire block (which can be large) and then rewrite it. Sometimes the zeroing operation takes a significant time.
Basically all TRIM is doing is when you delete a file, it tells the hard drive it can zero the blocks that were used by the file. This makes the blocks around twice as fast to write to later, because they're already cleared.
Simplest analogy is a chalkboard. It's faster to write something on the board if you don't have to get out the eraser and erase it first. This just makes a habit of erasing parts of the board whenever possible, so when you need big chunk of space to use, it's already ready to go.
