All SSDs have a bit more storage than their rating. Partitioning a little less space on a vendor-fresh drive can double or triple the extra storage available to the SSD's internal wear leveling algorithms.
This won't actually work - partitions don't exist from a disk's point of view, but are just bytes in sector 0. The SSD will religiously preserve the useless data in the sectors outside of the partition you create, using up space that could otherwise be put to good use.
As other posters have explained in bits and pieces, flash chips can be written in pages (2KB or 4KB, usually) but have to be erased in blocks (64 or 128 pages). If you overwrite 10 pages in the middle of a block, the writes will go into fresh pages somewhere else, and the original 10 will be useless until you get around to erasing the entire page. Since those other pages are holding data, you can't erase them until they either (a) get replaced by additional new writes, or (b) are moved somewhere else. If you end up having to copy say 3 pages of data for every new page you write, your write performance is going to go down by a factor of 4.
The more free space you have, the more likely it is that even with totally random writes there will be some blocks that are entirely empty and can be erased without having to copy any data. That's why the 32GB Intel X25-E (the enterprise drive) has 40GB of flash chips inside it. On the other hand, just about every consumer drive has 6.7% or so free space, because that lets them use say 64GB (64 * 2^30) of flash chips and legally advertise it as 64GB (64 * 10^9).
Typically your file system has a fair amount of free space (compared to 6.7%), because performance suffers and you run the risk of running out of space when you get close to 100% usage. Without TRIM, however, the SSD can't make use of that space, and carefully preserves the contents of every block on the file system free list. In theory TRIM should allow the OS to identify the file system free list to the SSD, which will then have much more space available for garbage collection, resulting in reduced copying and better performance. In practice, your mileage may vary.