Will,
First of all, congratulations on the save! You've already had some great answers to your question, but I thought I'd also weigh in with an answer.
I suspect that you were witnessing the pre-arrest phase, something which we don't tend too see that much as EMS providers. The weak carotid pulse does seem to suggest a form of (barely perfusing) ventricular tachycardia. The epileptiform seizure activity is a short pre-cursor to unresponsiveness as the brain is starved of oxygen. It's actually a pretty scary sight: I have seen it a few times myself, you just know you're in for some fast action..
A witnessed arrest is, of course, the best type to have. You were right on top of it and that's why the pt. became responsive again. His hypoxic episode was short and sweet, leading to a quicker recovery. My one abiding memory is of the 44 yr ols that arrested in my truck who was given a succesful pre-cordial thump. He was awake again in a matter of seconds and said: "Man, why did you have to go and hit me like that?".
Good call!!
WM