Usually, the more boring glitches aren't very fun to watch. That's why some TASes are more exciting than others. Otherwise, there are non-glitch TASes which are also fun to watch, and play within the games rules.
The info from the person who discovered the glitch is in
Post 15 by UncleSporky.
It's a little hard to explain. Basically, when you go to the game's Title Screen after death, "random" data is stored in the area of memory where level data would normally be. When you load a game, this random data is still present (provided the area you load doesn't overwrite the random data with real level data).
By dying in that specific room in Brinstar, the "random" data was written to memory, but it was specific level data that he wanted. So when he loaded the Tourian file, that data was still there. With that level data, he was able to skip Mother Brain by going in the ceiling and utilizing that data.
What was that specific data? It was door data. He created a door for himself.