In general there are 2 main forms of steel glitches in PC Lemmings:
1) lazy steel checking: the expected way things should work is that a pixel of terrain inside a steel area should be unaffected by a terrain removal operation, while other pixels lying outside it should be affected. Instead, the game takes a shortcut and basically does something akin to checking whether the lemming (which is essentially treated as a single pixel) is standing on a steel area (for diggers, exploders, and I think miners too actually), or has steel area coming ahead (for bashers). If the lemming isn't, the entire terrain removal operation will go through even if some of the pixels removed are covered by the steel area. Otherwise, the removal operation is either aborted, or will not take out any pixels, even ones that are outside the steel area.
Examples of how this translates:
- a falling lemming explodes just a little above a steel floor. Because he is above the steel floor rather than standing on it, the explosion will take out the entire usual area of explosion, including parts of the steel floor that overlaps with the explosion area. In contrast, if the lemming was walking on the steel floor when he explodes, the explosion will take out nothing, even if the lemming was standing next to a wall that's not covered by steel area (so one would expect those pixels of terrain be taken out, but no).
- a lemming just steps off the edge of a steel block and falls. This puts him just outside the horizontal range of the steel area, so if he explodes now, the full explosion area will be taken out, including those pixels of the steel block near the edge where the lemming is falling from. In practice, steel areas in the game are not often set up to exactly align visually with the steel terrain you see [more on that later], so you might need to starting falling a little further away from the edge for this to work .
- you have a builder lay down a single brick at the edge of the steel block, and once the builder steps up onto the brick, you make him dig down. Because the game treats the lemming as a single pixel for collision and steel-checking purposes, the fact that stepping up onto the build brick puts the lemming just slightly outside of the horizontal range of the block of steel will allow his digging to continue down the edge of steel block, taking out some of the pixels of the steel block. You can achieve something similar in the case where the level already has regular terrain next to the steel block.
- You can bash up to 7 rows of steel starting from the first row of pixels above floor level. This is because the game really only checks for steel starting from the 8th row up from floor level, and seeing no steel, it will allow the bash stroke to fully take effect, even on pixels that are covered by steel.
One further wrinkle is that due to technical limitations, steel areas in the game are not necessarily aligned with the terrain you see, sometimes it extends one or two rows/columns above the visible terrain's edges, other times it falls short by a small amount. You'll need to use a level editor (eg. LemEdit, Lemmix, etc.) to see the actual positioning and range of the steel areas.
2) blocker steel-canceling: the game implements blockers by setting up a "field of blocking" around the blocker. This field will temporarily cancel out any parts of steel areas (in fact, also things like trap triggers!) that overlap with the field.
A simple example to try out is Tricky 9. Have the first lemming out block when he lands. Then have the 2nd lemming out dig when he lands (to assign this lemming a skill, you'll need the technique of "walkers-only" selection, done by holding down the right mouse button while selecting and clicking on the lemming with the usual left button). You will see him dig down the steel even though he shouldn't. Of course, doing so also frees the blocker right away, thus ending the field of steel-canceling, so on the next dig-stroke he will fail.
3) there is also a very specialized form of steel digging using a very specialized glitch unrelated to the above two. See
this post for details.
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I don't remember Tricky 9's min solution, but most likely it uses #2 (blocker steel-canceling) together with exploders and #1 to blast apart the steel floor right below the entrance trapdoor. This then allows lemmings to directly drop to the exit. Due to a glitch in PC Lemmings, such "direct-drop" lemmings will exit instead of going splat. For Tricky 13, you start with the "build and dig down edge of steel block" trick explained in #1; the rest is glitch-free (but far from trivial!).