The Ten Second Warning is the brief period before vomiting where you are aware of what is about to happen. It's like your body telling you to get to a sink or toilet, unless you want the extra hassle of mopping your own stomach acid off the floor.
"Hmm, I'm suddenly wide awake; there better be a good reason for my precious sleep being disturbed. My stomach doesn't feel so good, and-- oh crap, it's The Ten Second Warning! Find a toilet!"