Took me awhile to figure it out. You hold space and pres "o" or "p" to look at the two maps behind and head. It will show the monsters and locations of items and trap doors.
You need to lure the monsters in to open trap doors, either by getting them to chase you, or by positioning yourself on the current map, so when you warp to the previous or the next, the change in map causes a trap door to open up under the monster.