What is ATA TRIM?
An SSD's total size is composed of thousands of smaller units called "blocks," which average about 512k these days. SSDs deliberately try to spread written data across all of these blocks so as not to prematurely wear out the memory chips, which can only accept a limited number of writes. This technique is called wear leveling. Over time, wear leveling guarantees that every block on the SSD will receive a write of data at one time or another.
When this is combined with Windows' delete mechanism, which only marks space as free rather than physically removing the data, an SSD is guaranteed to get gummed up with a hodgepodge of deleted and undeleted files. When the SSD's physical cells are full, regardless of the displayed free capacity, the drive must perform a complicated routine called the read/erase/modify/write cycle to store new data.
An REMW cycle forces an SSD to scan its blocks for deleted but unpurged files, copy active data to cache, purge the deleted files, append the new data to the data in cache, and then write the cache back to the new free space. This is called write amplification, and in serious cases, it can force an SSD to shuffle up to 20GB of data just to write 1GB of new information. This causes significant performance issues
The solution to this problem is to let SSDs physically erase files the moment they are deleted in the OS, and that is precisely what the TRIM command does. Windows 7 and Windows Server 2008 R2 are the only Microsoft OSes that support it, and the feature cannot be used without support from both the drive's controller and firmware.