Page propagation is triggered by publishing a page on the source variation site by default. Each time you publish a source page, the Variations Event Receiver adds a work item to the Variations Propagate Pages timer job queue. When the timer job runs, it will begin executing the first 100 page propagation work items. For each work item, Variations will copy the source page to all target sites, creating the page if it does not yet exist, or appending a draft minor version if the target page does already exist.
In some cases, users might not want changes to a page on the source to necessarily propagate to all targets. That is, users might want to make source-local changes and have the option to make changes globally applicable when they want. This often takes the form of a question like "How can I stop variations from overwriting my target pages every time I publish a source page?" Variations in SharePoint 2010 helps you do this.
The Answer is Enable On-Demand Page Propagation.
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