Most of the plugins work based on component Events (triggers). This is a Push mechanism. If an Event happens (E.g. a photo upload or a new article event), the content is pushed to the social networks.
In the other hand, there are cases where no Events are fired (a RSS Importer, adding new articles), or you can have previous content (before AutoTweetNg installation) to promote. So, you need Content Polling to check if there's anything to publish to social channels. This is Pull mechanism.