Well when an opposing player is in on a breakaway, it really depends on the situation. You usually know certain players tendencies, so with a guy like Nash you're usually anticipating a deke, but there are too many factors to say outright. Depending on how much time he has, how fast he's coming in, how he he's carrying the puck and also what I'm giving him based on how far out I'm challenging, am I matching his speed and what does he know of my tendencies. It really depends on the situation.
When he's in 1on1 with a D, I would be expecting him to try and deke through the D or to try and power his way though and if he can't beat him to the point where there is no obstruction between the puck and me, then he will try and use the D as a screen.
That's what makes players like Nash such a threat, he his the size and speed to power through, but also the hands and quickness to suddenly shoot through the D's legs when you're not expecting a shot. This is also part of the reason why his shot speed necessarily isn't as important. Nash is incredibly shifty for a big guy and his hands are fantastic, so even if his shot isn't Tarasenko level, he is good as surprising the goalie and putting the puck in hard to stop spots like the 5-hole or short side.
I guess my main takeway from Nash is that he is incredibly shifty for a big guy and can seemingly deke or shoot at any time. It's why when people talk about his wet noodle of a shot, and all things honest compared to the elite snipers in the league his shot is weaker, I get annoyed. He is very shifty and he can shoot or deke seemingly at all times when he's in on goal, different from a player like Stepan who has an above average shot but telegraphs what he's doing pretty often so when he scores on an unscreened shot, it has to be 100% pinpoint accurate.
Nash gets compared to Tarasenko and Ovechkin and players like that because they're goal scorers and so is Nash, but he's different type of player. I've said it too much in this post, but Nash is a shifty player who doesn't rely on his howitzer of a shot, that's not his game. He likes to come in with speed and put the D off balance with pace changes and quick hands. How many times have we seen Nash put a D and goalie off balance and score on what looks like a noodle of a shot? That's not because his shot is so bad it's going in, it's because he knows what he's doing. With his speed and size and the momentum behind him, he's indicating one thing, then he shifts pace and either cuts back or he makes a small move like slightly moving the puck to either side before releasing his shot or holding the puck a millisecond longer to change the nagle slightly which is very deceptive. The way a player's stick is lying as he's about to release the shot is what the goalie is reading to anticipate the shot.
I'm on the train, so my thoughts are a little jumbled and this is a pretty rambling and incoherent post, but here's a few Rangers Nash examples of what I'm trying to get at.
Here is Nash holding the puck an extra second longer, changing the angle slightly on Rinne to beat him short side.
Nash making a slight deke before releasing the shot with great accuracy
Nash first goal in this video, right before he releases he rolls the puck slightly off his heel.
We all know how good he is at faking the goalie out using his forward momentum and suddenly changing direction and his great hands in tight
The way Nash pulls the puck back for a quick deke and changes his angle before releasing completely fools Bryz
Nash holding the puck an extra second longer:
Nash great skill here