Tip: Switch palace switches normally disappear if you replay the level. However, if you insert them as direct Map16 and use the hex edit at $00EEB2 in the ROM map, you can replay the level without a problem.
I think he means if you get pushed off in the wall when duckjumping under the ceiling, you'll get stuck in it and if you jump, you'll get pushed down (or somewhere else, I don't know). Not sure if it's right but at least something like that can happen.
#68 basically means when your big, and you duckjump to the top left of where it's blocked by a ceiling and you basically like doing a walljump except you don't jump, once you catch the wall at the same time when your blocked by the ceiling, than let go of down and you'll go through the wall.
Ill try and find a video of it, cause i've seem people do it before.