(As written in the N in-game help:)
"These little ledges will only stop you in one direction. Hopefully it's not the direction you want to go."
As the name suggests, the ninja is blocked by this object in one direction. The ninja can't pass the platform going the other way. Drones and other enemies are not affected or restricted by oneway platforms, so they may be used to create areas where enemies may go but the ninja cannot. These can also be used to trap the ninja, and forcing the ninja to commit suicide.
Glitch[]
There are many glitches regarding oneway platforms. There is one glitch that allows a ninja to move through downward-facing oneway platforms. When a ninja is walljumping, the collision system doesn't check to see what's above the ninja. So, if you get right up by a oneway platform next to a flat wall, you can walljump through the oneway platform.
This glitch is often exploited in level 20-1: Nexus, where highscorers use this glitch to get the gold in two chambers instead of one, increasing the score.
https://archive.is/20131014201619/img144.imageshack.us/img144/9994/boptu5.png
- 20-1: Nexus