I'm thinking of pull at the moment is 'creating an opportunity to get what I want' and push as 'imposing what I want' but that seems a bit extreme.I'm intrigued by this, and having recently played Polaris with my group, I am thinking in terms of its operations.
So in Polaris conflicts, there is a key difference between (1) "But Only If" and "And Furthermore" and "And That Was How It Happened" versus (2) "You Ask Far Too Much" and "It Was Not Meant To Be" and "It Shall Not Come To Pass". The former are additive -- you don't contradict your opponent, but rather agree with her. The latter are negational, denying what the opponent has said.
To me, the idea of pull brings to mind a conflict which only uses additive phrases. Improv theater games generally work this way -- the key rule is "don't contradict". The same is true of most freeform fictional games. The idea of push brings to mind a conflict which uses only negational phrases -- contradicting another player, or narrating in things which you know another player won't accept.
This also makes me think about the newsreels in Lee Short's Star, Moon, Cross -- which we used in our Buffy game. That is also a purely additive mechanic. Each player makes an uncontradicted assertion. Others can elaborate, which may pull the whole in a different direction, but they cannot negate what was previously said.
I'm not sure if this fits with how other people were picturing the concepts, though. Additive techniques demand that you stay within players limits, whereas negational techniques you run past the limits and are reigned in. This seems intuitively like push and pull. I think it also matches Mo's original association with gender -- that pull and additive are more feminine, whereas push and negational are more masculine. Still, I'm not sure on this and would be interested in other views.
A player asserting intent (like "I try to break into the fortress") is generally additive, compared to asserting effect (like Soap or Polaris where you state what happens). In the former, you demand an answer from another player (i.e. the GM) to state effect, but you're not demanding what it is.
Push may depend on having a mechanism for overruling another player's assertion. Some games don't have this -- I'm thinking of things like theater games, strict-rules larp, and final GM authority in tabletop. I was intrigued by Neel Krishnaswami's game posted to the 20' By 20' Room, "The Court of the Empress: an RPG" -- and by Mark Woodhouse's response to it that it was ruleless. Actually, it is almost pure rules, but they have no uncertainty or resource spending. There is no room for push in them, I think.
In the thread on Vincent's anyway, Tony Lower-Basch wrote:
It seems to me that you're combining our intuitive notion that we wholly "own" the characters ("This is one thing because it's being instigated by me on your character, that is another thing because it's being instigated by me on my own character") with our intuitive notion that we wholly "own" our actions ("This is one thing because I did it, that is another thing because you did it") and getting a very seductive but not really true distinction.This is interesting. Additive versus negational is a purely objective phenomenon. That is, are player's statements kept as true? On the other hand, this is related to the issue of ownership. That is, where are players' limits and how do we respect them? (i.e. What is acceptable in games?) Where players put their limits is defined by how they identify, and is affected by the rules they play under. I will tend to identify with a character more if I have more control, but that's not absolute.
Update: Four more related posts on the topic...
Jonathan Walton in the 20' by 20' Room on Push/Pull, Yin/Yang, and All That Jazz
Brand Robins on Brand Pushes and Pulls and Blows Himself Down
Mo on "Pull Clarification and Promises! Promises!"
Nick Wedig in rpg_theory on "Applying push/pull to play"