As above it just gets the new potential subscriber an extra 2 weeks free trial (4 weeks total) over and above the 2 week existing trial period. If you subscribe for a year you get the 12 months for price of 10, so $50 instead of 12 * $5 - no need for a referral to take advantage of that.
For the person referring, its a $5 credit on their account if the person takes up a paid subscription. No credit if they don't. I've had a few referrals and a large proportion didn't end up continuing after the trial. The ones that do subscribe are a nice bonus and its kept my TeslaFi costs averaging below £20/year so far. So thankyou to those that have used my code and carried on subscribing.