The L7 Room is a room you have to pass through in Great Palace in order to reach Thunderbird and Dark Link. It is one of very few rooms in the game that enterable from both sides, but does not allow you to exit on the opposite side as you entered. Exiting through the bottom takes you through a point of no return into the boss area of the palace.
The GP Alt-Route takes you to this room on the 'wrong' side, from which you are unable to get to the hole in the floor that takes you to the bosses. This is a frustrating dead end to find for new players. However, using a damage boost or precisely lined up downstabbing on the Ras that fly through the room will allow you to exit on the right and re-enter into the bottom section, thus allowing you to use this route to get to the boss area and complete the game, potentially without the Downstab technique.
(todo: someone fill this out with a text description? See Eco Key for a good way to describe it)
See the video linked in #Resources for a video description.
(todo: someone fill this out maybe? Even if it is a lot less used)
(todo: explanation of how/when this is applicable in rando)
- chromataclysm's video describing the damage boost