Performing the clip
PAL Clip is performed by doing a wall clip on a specific wall outside of the TR-2 Building in Chain Reaction. When you get through the wall, you will fall out of bounds. Simple move forward and left and crouch in the air and you will land on an invisible floor. At this point you are inside of the building and all of the collision is loaded. Only the textures are not loaded. Navigate your way to the stairs at the end of the room and the textures will load. If you can't find the stairs, simply keep moving forward and the textures will eventually load.
History of the glitch
A few days before PAL Clip was discovered, Veman3000 found a way to clip inside of the TR-2 building by placing a ronin and using it to get clip storage. So at this point there were two different tricks in the same level that used the Ronin. Ronin Storage (mentioned above), and Ronin Jump. Ronin Jump saved far more time so Ronin Storage was not used in speedruns until a few days later when Veman3000 found a way to re-collect the ronin after doing Ronin Storage. Unfortunately, this glitch was extremely difficult to do and not very consistent. It was very easy to make a small mistake and lose a lot of time because of it. On November 20, 2015, Kykinson uploaded Chain Reaction in 1:57, the first sub-2 time on the level. This was also the first recorded use of PAL Clip. At the time, PAL Clip involved clipping through the wall into TR-2 and placing a ronin inside the building. Using a ronin while you're out of bounds will reset Bond's height so you were able to get back inbounds.
This was the standard way to do PAL Clip until a new, faster method was discovered on January 27, 2016 by FitterSpace. The new method involved clipping through the wall and landing on the seam just outside of the building. Then move to the end of the seam and jump on to a specific area to land inside the building. This method was faster because you did not have to place the ronin or re-collect it when you get back inbounds. Soon after, Canariabird found the current method of getting back inbounds which saves even more time.