In my opinion the game of push hands in order to be more than a watered down version of grappling( this can be fun but in my mind is not the point of the game) must contain the following if it is to fulfill its purpose of skill development.
1. Sticking - do not break from the point of contact, it is the ball and must be honored and manipulated by changing the body behind the point of contact as opposed to the point of contact itself.
2. Asking - you must be offering something to your opponent or else there is no game, in other words you must be actively seeking their center and outreaching to have an effect.
3. Maintain structure - this is an extension of the first point. If you collapse and change structure in an effort to alleviate the pressure being felt you are changing conditions as opposed to transforming them.
These three points allow for a circuit to be established between the two players and it is the manipulation of the pressure that builds up within this circuit that strengthens and transforms the body, breaking any of these three conditions is a pressure release and negates the ability to internally absorb, transform and release the energy back to your partner, which to my mind is the point of the game. If the external conditions(3 points listed) change, the ability to develop internal change is compromised because there is no structure or container in which to move. The internal stretch requires external constraints in the same way that one needs to anchor a sling shot or elastic if it is to be stretched and released.
I write this not out of dogma but in an attempt to clear up the conditions surrounding a practice that seems to have so many definitions that it can be difficult to develop the skills intended because of the lack of coherent method and an agreement upon the game. It is difficult to get good at tennis if your parter is applying the rules of basketball. It is not that one is better than the other, but a thing is a thing in accord to the rules that bind it and there is no need to put legs on a snake.