In general there are 2 main forms of steel glitches in PC Lemmings:

1) lazy steel checking:

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.

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!).

Quote from: ccexplore on 2010-09-22 05:25:51

Oh right, so there you go, a different method exists <slaps forehead >.

To start with, I placed the blocker as soon as it landed, which pushes lemmings into the wall (thus introducing a needless glitch). So there you go, I get to slap my forehead too

To maximise your chances of success, you should dig down as far as possible while still freeing the other groups (9 pixels), and also as far to the left as possible. This helps to keep the lemmings in tight groups.