Undefined and up to the DM. This is a feature, not a bug!
In AD&D, both editions, magic items are the exclusive domain of the DM and they are given full rein in deciding how they work, which is to say:
- If you're the DM, the game doesn't care, so make up whatever sounds good.
- If you're a player, you have to ask your DM, or (more likely) experiment with the item in-game.
This is a deliberate design feature of AD&D. It means that players of the game can argue all they want about how it works, but when it comes time to sit down and play the game, arguments don't impede play because it's the exclusive call of the DM. It's also a feature because it's a prompt for creativity: for decades, AD&D was full of magic items with lacunae like this for the benefit of creative players and DMs to customise their gameplay. These lacunae are meant to be explored: they enrich your campaign beyond anything that could be written by a designer miles and years distant, and the consequences of how a DM's answer fills in the blanks creates opportunities for experiment and exploitation.
Because of this difference in philosophical approach between AD&D and D&D 3.x, there is no way to fill in this kind of descriptive gap in an item in an official way if the item hasn't been rewritten officially for 3.x.
Consequently, a 3.x DM must import the AD&D philosophy for just a moment, and make something up. A 3.x player must do similarly, and wait for their DM to make something up.
Personally, I can see several ways I could rule this, and each have very interesting consequences, either in terms of danger to the PCs, opportunities to exploit the feature, or ways to kick off entire unexpected adventures. But I will refrain from sharing them in a big opinionated list, because that would defeat the purpose: your group's DM should have fun answering this question, and your group's players should have fun directing their PCs in investigating it.