The One Way Nether Portal is a single-use item that not only sends the player to the Nether without a return portal, but also sets the player's Spawnpoint in said Nether. It disappears immediately after the Player interacts with the Overworld in any way. Of course, since you installed this (hypothetical) mod, you'll most likely use the Portal. The Portal cannot be re-crafted, so choose wisely. The disappearing act it makes when you punch a tree is so that the mod can blend with vanilla Minecraft, giving the player the option to play vanilla and having another save in the Nether without de-installing the mod. (Features in the Mod are not available if you construct a classic, two way Portal.)