Fast swimming is a technique that allows Jak to swim faster than usual. It is performed by tapping the analog stick forward and allowing it to return to neutral in a regular rhythm.
Jak's swimming animation consists of a section where he moves fast, followed by a section in which he moves more slowly. By repeatedly tapping the analog stick forward and allowing it to return to neutral, only the first part of the animation is completed, allowing the player to use only the fast part of the animation repeatedly without the slow part being used.
It is not currently known how or by whom this technique was discovered, though there are records on Cheatcc.com noting its existence as early as 2003.