Note: This page is taken from n.wikia.com.
(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 one way platforms in one direction. The ninja can't pass the platform going the other way. Drones and other enemies are not affected or restricted by these objects, 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.
In the 2.0 version, one can go through a downward-facing one way platform. To do so, the player has to be lifted up by a thwump and jump right before hitting the platform.
Diagonal one way platforms can also be made using coding.