It works because it frees up clear blocks of space on the SSD for sequential writes.  Nothing to do with reads.  Benchmarks here if you don't beleive me:http://www.ocztechnologyforum.com/forum/showp ...

I beat you to it I'm afraid - look further down the comments :) ...

Sorry guys but you are wrong on the SSD and defrag front.  It actually does help an SSD to defrag the drive as it enables more writes to be sequential rather than random which increases performance. ...