The Dork Knight himself.
Quote from Inexistence:
In the map, there is a small point the map creator needs to put in that will trigger a map change. Sometimes, this is a trigger, so you have to walk into it and the map will change. However, sometimes the map maker will want the map to change on something happening, maybe a door closing or something. In this case, they still need to make an entity to change the level, and put it somewhere in the map. It's not a trigger, but it's still there.
Spider-Waffle has found that if you touch this entity, it will change the level, even if it isn't a trigger. So if we're waiting for some event to happen in a map before the map changes, then instead we can just touch this trigger and have the map changed. I'm not sure if this applies in the original half life, but in half life: source, this is the same, and in the test chamber near the start of the game, very high up, there is the map change entity, and if you manage to jump up to it, you can change the level, even before anything has actually happened in the chamber.
Hope that covered it.
Spider-Waffle has found that if you touch this entity, it will change the level, even if it isn't a trigger. So if we're waiting for some event to happen in a map before the map changes, then instead we can just touch this trigger and have the map changed. I'm not sure if this applies in the original half life, but in half life: source, this is the same, and in the test chamber near the start of the game, very high up, there is the map change entity, and if you manage to jump up to it, you can change the level, even before anything has actually happened in the chamber.
Hope that covered it.
Well that explains lvl change ents. The burning question is how high up is it and can it be reached with no weapons?