His strength had merely been 16 points before advancing to form 6, which means 2 points had been added.
Agility was at 12 points, increased by 2 points.
Vitality was also at 12 points, also an increment of 2 points.
Intelligence was the same, so there were no changes.
For the potential points to have silently fallen to 0, Garen believed he had a theory; after killing Hellgate, he didn't obtain any potential points, hence he didn't have any increments. Whereas ever since his closed training, the potential points had decreased during the process of recovery over the years. Obviously, the recovery of the body via Secret Technique was due to the consumption of the potential points.
Even though the Secret Technique absorbed the pressure and extreme cold to repair the body, the source of the power that was used to activate the recovery was ultimately the potential points.