In some respects I don't know, as part of this tech is new (The part which allows voluntary movement). I have a feeling the patient only had a partial spinal cord injury (Though, usually with partials there is some movement retained). In a partial, the nerves themselves aren't severed, so it would then seem like the stimulation allowed the normal nerve impulses to travel over the part which was damaged and make it down to the nerve.
Either way, 'stimulating' a nerve means applying an electric current to it to simulate a brain-signal. It's not as complicated as it sounds, it's not much more then sticking an electrode around the nerve and then applying an AC or DC current (Though AC and DC do actually produce slightly different effects). I've had it done to my hand before (Via some simple surface electrodes), depending on the placement and nerves which receive the current different muscles start pulling. If you stimulate the correct nerves, you can have the correct muscles pull and create Ex. a fist, or a clenching motion. Implants right now don't do a ton more then a simple 'open-close' type motion for a hand (Since the intent is not so you can type on a computer, but so you can eat by yourself or pick-up a glass, etc.)
Either way, 'stimulating' a nerve means applying an electric current to it to simulate a brain-signal. It's not as complicated as it sounds, it's not much more then sticking an electrode around the nerve and then applying an AC or DC current (Though AC and DC do actually produce slightly different effects). I've had it done to my hand before (Via some simple surface electrodes), depending on the placement and nerves which receive the current different muscles start pulling. If you stimulate the correct nerves, you can have the correct muscles pull and create Ex. a fist, or a clenching motion. Implants right now don't do a ton more then a simple 'open-close' type motion for a hand (Since the intent is not so you can type on a computer, but so you can eat by yourself or pick-up a glass, etc.)