Upon triggering, the floor in the affected area collapses into a chasm, except for a very narrow balance beam in the center. Characters may roll Dex, Saving Throws, or use some sort of acrobatic skill to grab hold of the beam before falling (GM's choice). After this, walking across the beam will require more rolls for every ten feet traversed. Strong characters may use Strength instead, and shuffle along the beam hanging by their hands (still needs a roll, though). Certain magics may help here.
Falling may mean automatic death, falling damage, or the chasm may simply lead to another level of the dungeon, at the GM's discretion.