I'm going to state how I think the select glitch works. This is all based on casual observation, so no formal testing has been done on this.
Anywho. We're all familiar with what happens when the screen buffer is poorly implemented and is the same size as your screen. The tiles behind you end up getting placed in front of you. Usually this isn't visible as the buffer is often larger than the screen, but it can be seen in some games.
This is basically standard practice. It's very, very safe to assume the same thing is going on in this game, but you can't see it as it takes place off screen (like it should). As you're moving along, the game will drop the parts that are left over, load what should be there, and "place" it. I mentioned before that switching weapons holds the game up; my guess is that if you press select at the exact moment the game is supposed to change what's ahead of you, the pause causes the programming to miss the "drop and load" part and skip straight to the "place" step. Voila. What was behind you is now ahead of you, including
empty spaces. if you can place an empty space on the edge of a room, secret worlds pop up.
How does this account for some secret worlds, like the one underneath Samus's ship? After all, there are no pits in that room. Here's my answer: I don't know for certain. My guess is that it may have something to do with the presumably intangible blocks beyond the left side of the room, since I've only mangaged to access it while approaching the ship from the left.
Confuzled yet?
Edit: Okay, not only am I now conviced of the "placement" thing, but after breaking into a secret world to the left of the ship, my suspicions have been confirmed: the secret world under the ship is the result of some junk tiles outside of the room that can't ordinarily be seen.
It would also seem to only work if the screen is moving faster than two pixels per frame, aka turning around or falling. Bit of a shame, really, as this limits its usefulness.
Well, using the properties stated above, I used the select glitch to move parts of the shaft above me to remove the bottom. It took four changes (two apiece), and has given me direct access to the spider ball without having to muck about in that maze. You'll still have to use the maze to come back out, though.
You'll notice the ball looks different than it should; yes, I used a Game Shark code to give me stuff to make it easier to explore secret worlds.