The usual explanation, if you press them on this issue, is that they are guided by reading the inspired scriptures.
Of course this means that they are as guided as anybody else who reads the scriptures as well, and this is why they hate breaking down this issue too much. They much rather have this wrapped in a thin veil of mistery and fuzzy logic, and to be fair, they have done a very good job at it. Most JWs still think they are inspired or something very close to it, despite the fact that they admit they are not.
When i was an active witness, i had this idea that when the GB met, it was a mystical event where angels or some other spiritual force would join the meeting and tell them what to do. This admiration of something they don't quite understand or know, is essential to keeping active JWs hooked. In a recent discussing with my older sister, she admitted that the reason why she believes the GB, is because they are inspired. Imagine that! The very reason why she is willing to obey them is invalid and openly admitted by them. Misinformation is only helpful for the Watchtower.