It's never one thing, I watch so many inputs. I had made a post the other day about that subjective gut read of large amounts of data. Some people try to more strictly quantify a model with many inputs, I have always stuck to my intuitional read of many inputs. I watch individual names in certain sectors, counter asset movements, the vix, moving averages, volume, media as a contra indicator (including this thread). More than 50 different inputs that I study.
If you want to hone in on "one thing" for this one. Load up NFLX chart and take look at 11:20 till 12:30. There was a sentiment turn on an insane flush. This setup in NFLX on the 5m is by far my favorite pattern, classic double tap that doesn't go lower and recapture of previous hump. I'm like BUbbles in a room of 12yr asian boys, I just can't help myself on a setup like that. I flicked to several more names to check that I was satisfied they had hit my flush out goals and then jumped in.