The reason I used the event horizon analogy is that as the sensory input becomes less and less, I get the feeling that I'm 'falling through realities' at a faster and faster rate until there's no distinct change between the different realities I'm experiencing. Once it becomes a smooth, continuous change in reality, with no feeling of ever stopping in any particular 'reality', then that's the 'crossing the event horizon' moment into the hole.
Funnily enough, it's not that process in reverse when you're coming out/emergence; it's pretty much a steady state reality but you slowly feel the sensory input returning, followed by the feeling of them all integrating with each other. Possibly the weirdest, most intruiging drug I've ever taken.