When go through your backlog, like any inbox (including a "main" one, your base field of operations for executing the GTD algorithms), you try to either take any needed action immediately (but only if it's short; perhaps set a threshold in seconds, rather than minutes) or classify for later doing.
My take: as I can, I try to expand items, breaking them down into simpler actions and tagging them (classifying them) with a greater degree of specificity. This makes the tasks more tractable over time.
You classify according to [[time]] and [[context]] for the task of doing. Some example tags: