I can only make a guess. Put the weed in a bag or whatever container and keep filling it up and up and up until you can't possibly squeeze another bud into it. Press and push as hard as you can until you simply can't fill it any longer. Now you have compact bud that hasn't lost it's trichomes (well, except the bud that's touching the container).
I think the reason to compact would be to conserve on space or there is a limited number of containers compared to the amount of weed.