It's not one sided like that, though. The players are also drawing things that force the DM to adjust his drawing, so it's a shared drawing experience.
Example. DM draws an ogre, thinking that there will be a fight. Players see the ogre drawing and the party illusionist casts a spell drawing himself as an ogress and begins to speak to the ogre. Now the DM and player jointly draw an ogre interaction instead of a fight.
That's how Story Before goes. It's a joint art project, even though the DM has backstory. Unless the DM forces the backstory(or parts of the back story) to happen no matter what the players try and do, there is no force or railroading going on.
Well done Story Before features a lot of things that ham-fisted Story Before does not (my response to prabe which I C/P above about railroading vs not in Story Before attempts to capture that).
Also, there are a lot of folks (I sense you're one of them...ER is clearly another) which have some kind of synthesis of Story Before and Story Now happening simultaneously.
But in the above analogy (for explanatory power I'm examining at the zoomed out concept level), I'm just trying to cite a particular instance of play for both Story Now and Story Before, not the entire composite of play (eg from Session 1 all the way to Session Donezo).
For that (one unit of play), it basically looks like this formulation:
STORY BEFORE (as Pictionary)
Unrevealed backstory > GM draws picture > player's make inferences/solves > players either solve or they don't based on a composite of (a) GM skill in drawing and (b) player skill in inferring and integrating
(In Story Before play, backstory now revealed can then be relevant to subsequent player moves or GM situation framing...or we can move on to a new section of play where play moves to a new suite of unrevealed backstory and rinse/repeat above)
STORY NOW (as Telephone)
Inciting thing > filtered through snowballing procedure (for Telephone that is a chain of whispering and listening) > evolved and established thing
(In Story Now play, this evolved and established thing can be any/all of character/setting/situation...we learned about Bob, we learned about Bob's hometown, we learned about the clustereff relating to Bob's brother and how that fallout is haunting us presently)