Wall-Clipping is a method used mostly in speedruns (and by some casual players on Crushing difficulty) of Uncharted: Drake's Fortune (and one used in speedruns of Uncharted 2: Among Thieves). There are two wall-clipping methods. The old method was to crouch down behind cover and move the analogue stick used for movement left and right in a rhythm, clipping to the right of a wall/object and the newer method is done by spamming L1 or L2 and moving South-West (if Nate was facing North), clipping through almost any vertical surface.
Uncharted: Drake's Fortune
The second method is used for most of the OOB (out of bounds, out of level areas) skips in Uncharted: Drake's Fortune, while the first method is still used to skip triggering the opening fight in the chapter Trapped.
Uncharted 2: Among Thieves
In Uncharted 2: Among Thieves speedruns, the wall-clip method is only used once, at the beginning of the chapter Locomotion, where runners clip off of the train, and wait for the train to come back, and by jumping to the beginning of the train, they skip most of the chapters Locomotion and Tunnel Vision. Most of the skips in Among Thieves are done by airwalking (repeatedly pressing the inspect button, only works on non-updated disc versions of the game).
Speedruns of Uncharted 3: Drake's Deception and Uncharted: Golden Abyss are mostly (if not, entirely) glitchless. Besides some OOB areas/skips, one of the most useful skips in the game is in the chapter The Chateau where a "camera launch glitch" is used to unload an area, load up another and trigger a checkpoint which can be restarted at by restarting from checkpoint. The glitch starts in the area where you re-find Sully (after fighting Marlowe's agents and completing the grenade tutorial) and ends after the cryptic puzzle underground. The camera launch is triggered by repeatedly pressing the inspect button and eventually stopping after a while. Golden Abyss speedruns are glitchless.