Now before you attack me with all the Valyrian steel in Westeros, I just want to point out that I am merely a messenger, delivering hypothesis posed by others for you to make what you will of them.
The views represented below are not neccessarily my own but they are worth considering.
After watching 'The Long Night,' it's safe to say that everyone was bit disappointed in the performance of Jon. Indeed, on our player ratings podcast, we gave him what we believe to be a generous five out of ten for his sub-par display in the Battle of Winterfell.
His belligerent, mé féin tactics were destined for failure and he lack of wit was typified in how, when it mattered most, he seemed to throw in the towel and decide to just scream at Viserion, the undead dragon. A hopelessly stupid act.
Or was it?
Many Game of Thrones fans are now arguing that Jon was not merely screaming for the sake of it. Instead he was distracting Viserion and yelling at Arya to make a burst for the Godswood. There are two parts to this theory. Firstly, it's proposed that we can see Jon slightly tilt his head after noticing Arya (0:27), and secondly, that when he jumps up, what he actually screams is "GOOOOOO! GO! GO! (1:30)"
After rewatching the scene, with headphones on (an important tool) it certainly does sound like Jon is screaming 'Go.' If also fits perfectly timewise, as the next shot cuts to a white walker feeling a gust blow by as Arya entered the Godswood.
However, I'm not for a second buying the arugment that Jon's head tilt is him noticing Arya. I would not be suprised if it did transpire that he was potentially sacrificing himself by distracting the dragon for his sister/cousin but the more likely case is that we're just a bunch of excited fans clutching at straws.