In Angry Aztec, you normally need to open the huge door to get into the second tunnel, which Diddy is supposed to do with his guitar on top of the llama cage. This trick lets you get out of bounds and walk into the tunnel without opening the door.
In the first main area, walk into the right corner behind the temple. Make sure your kong faces the wall behind the temple and start backflipping into the corner. Slowly inch your way into it, and when it looks like you're partly inside the right wall, start inching up toward the other wall. Don't just press the control stick up; you need to approach the wall by pressing up and slightly left. If you land on the slope above and immediately fall down through the wall, or if you notice you're about halfway through it, you're ready to just walk forward and end up out of bounds.
From there, stay relatively close to the wall and walk toward the entrance of the tunnel. When you're just about next to the tree on the right, you can turn left by about 45 degrees and walk straight ahead to save some time.