Was on this evenings flight CUN-ORD, sitting in 5E on the 738.. About 2/3 through the uneventful flight an FA comes streaking up the aisle past us yelling "we need help in the back"! I leapt up and headed back where a crowd was gathered around row 22 or so. A middle-aged Mexican man who was struggling (slamming hands/arms up and down) and speaking somewhat forcefully was being restrained by pax in row behind, his wife was literally sprawled on top of him, and a big guy in the aisle. I was handed the plastic flexi cuffs and we tried to get him settled down and cuffed. Took awhile, but eventually after taking some sort of 'powerful antipsychotic' (per an on-board psychiatrist) Rx that his wife retrieved from the overhead he was cuffed and a few larger-type pax were seated behind and next to him. I returned to the front. The FA had a bit of a swollen lip. From what I could gather the pax had tried to light a cigarette and when the FA intervened he lost it, apparently suffering some kind of mental event. Pilot declared an emergency, and said sternly that if there were any more issues at all we would divert to Indy. We did not and were routed directly to ORD - fastest approach I've ever experienced - where we touched down about 40 minutes early and were greeted by numerous Chicago PD, a couple FBI-types and some Customs. They escorted the fellow off immediately, then began a rather lengthy interview of witnesses in the back. After about 20 minutes the let first class only leave as there were clearly no witnesses up there. I don't think there was much fear among the pax as he wasn't near an exit and was well restrained by his seatmates, but my first experience in this type of inexplicable event in the air.