So, for those who are wondering just what happened… there’s a glitch that’s been known for a while, where Yoshi can end up in the “I have an item in my mouth” state, but not actually have an item in his mouth. When he spits out this nothingness, the game crashes. That’s all we knew about the glitch… until recently.
Somebody decided to take a debugging emulator and step through the crash. He discovered that the crash occurred because the game tried to jump to a specific address in memory, and execute code there. That address did not contain code, and so the system crashed.
But wait a second. What if, by some sheer coincidence, that address did contain code? The specific address dropped him in somewhere amongst various data for the game’s internal random number generator, and the random number generator can be manipulated in a TAS. Could the game be coerced into running arbitrary code?
Well, as it turns out, it’s extremely limited. You have the RNG state and a few sprite positions that can be manipulated, but after that is some internal game stuff that can’t be. So the length of the “program” is extremely limited, but as it turns out, it’s just long enough to send the signal to the game to switch to the “The End” screen.
There’s a related glitch where you can spawn a bouncing fish by doing certain odd things as you spit out an item as Yoshi, and every time a fish bounces, it uses up a randomly-generated number (since it bounces in a random direction). So, putting it all together, you have to spawn a billion fish and have them all bounce a jillion times to get the RNG in just the right spot, leave the area, and then fiddle around with some objects to get the last few bits you need in the right place, all while setting up the original spit-nothing glitch in the first place. (Source