Pushing, as far as the term makes sense, has to be an overall attitude toward game play. People tend to given visual cues of reality when trying to innovate in matters of words, so my best guess would be that pushing qualified, initially the player's striking action. Pushing instead of striking... That would be doing the bare minimum to keep the ball alive, purposefully avoiding to hit hard, or even limiting oneself to bunting the ball back if it is possible.
In many discussions on the net and on courts, I have pictured pushing as an under-optimal playing strategy: that is, it's the defensive counterpart of ball bashing. Ideally, you do not err below, nor higher than necessary: objectively, risks can be rewarding when managed properly and exaggerations can hurt. Either way, it's a mistake because the response must always be adapted to the circumstance.
So, who's not a "pusher," but the exact opposite? People highlighted it properly here: a ball basher, someone who constantly tries to impose his presence with force and power, who tries to hit winners every two shots... that's a ball basher and that's the exact opposite mistake.