Essentially they just placed a log, stone, body or some such obstruction in the path. You naturally alter your stride to step over the object. This places your foot in the right place at the right time. Animals are equally vulnerable to this. Almost all of our figure fours are made with willow and discarded after they've been successful.
Animals smell the crushed gook on the traps.
Once you know the carving tricks it takes about five minutes to fashion a good figure four from one piece of willow. You may have trouble lifting the deadfall. In this case you can build a pile of stone on a primary rock once the trap has been set. This means that if you have good sets in four locations, one of your traps will be successful. Snares are another story.