It uses the miner positioning glitch such that the crowd can be trapped and released. I'm pretty sure the replay had been posted here. You should bookmark that page.
Sidenote: this is one of few results I've actually verified on Mac Lemmings. Although on the Mac, if I recall, some subtle differences with the terrain removal mask for miner means the 17/21 solution needs to be modified slightly to work.
I knew that this glitch was possible on the Mac, but I've no idea how to sort of activate it. Is it something to do with the bomber in this case...? if so, it might be something slightly different in that case – certainly on the Mac you can get it to kinda drop through a one-pixel gap on its own sometimes if you assign it in the right place.
The
glitch thread should have a reasonably good explanation for the miner glitch, and yes, the bomber is involved in the miner-glitch usage with the PC's 17/21 solution:
The miner's explosion countdown reaches 0 just when the miner positioning glitch places it, just before he starts mining through, below the bottommost row of pixels, so that he's actually standing on thin air when he starts "oh no". And since oh-no-ers fall, he never gets a chance to explode, and you have your crowd trapped.
But actually, I have to rework the 17/21 solution a little for the Mac, using a move that doesn't work on the PC version:
Because of subtle differences in the miner's terrain removal, the PC solution would've lost one more lemming on the Mac, due to that lemming being able to get out from the end of an in-progress miner tunnel. I don't remember the details off top of my head, but basically it forced me to rework the solution to use the bomber for some other purpose. So I make use of the miner positioning glitch by assigning digger to the miner at the same time the PC version's trick kicks in, for same effect (ie. miner falls through w/o breaking through the terrain). It doesn't work on the PC version because the 1st-time dig also takes out 2 additional rows of pixels above the expected 1 row below the lemming, whereas on the Mac version even the 1st dig takes out only 1 row of pixels (which is nothing in this case because the miner is already positioned to be standing on thin air).
Also... climberless Havoc 4 solution??
That's another one of those "simple solutions made possible by release rate jiggering" solutions. Paving the path is as simple as dig, bash, build. And the technique to work out how to make sure everyone exits in time is fairly similar to what I did for the min-skills solution for "Just a Minute".
This post describes my technique for working out the release rate jiggering with help of Lemmix's text versions of replays.
You should be able to find a replay for Havoc 4 somewhere on the
min-skills challenge thread if you want the exact details of the RR changes.