Without any physical alteration, a part of your brain that you don't usually use is converted to data storage. If you sacrifice some unimportant memories and social responses, capacity can be increased even further. This makes you a perfect, almost undetectable data courier. A Mnemonic Enhancer is always slick.
It's not much of a bonus to you while you're doing it, however. It interferes with your ability to emote and dream soundly, slowly eroding your sanity. Thankfully, the process is reversible, though lost memories must be retrieved from storage or they will forever remain hazy.
You cannot remember your past or use the Knowledge element of Skills to remember things more than a few years old. When the Mnenonic Enhancer is overloaded (as it will inevitabley be on exiting courier missions), your Mind and Chi are halved. Besides the obvious disadvantages, this makes you much more vulnerable to Stunts.