Triple Slash Clip (TSC) is a technique from OoT3D used to clip through walls. When Link performs a triple slash (press B 3 times in quick succession) and puts away his sword, he jumps backwards an immense distance, allowing us to clip into certain walls.
Link can only clip into walls with an acute angle. This is because the angle of the walls allows Link to recoil backwards, and not away from the wall. Some clips are easy (like the one from Termina to Observatory), and don't require any timing for the "Put Away" input. Other clips (like Snowhead BK Skip) are much more precise, and require timing the A input.