The idea behind document segmentation is that you produce a qvw that has summary data for all periods and detailed data for only a recent term.
For example, a sales app may have Sales numbers summarized by Region, Customer, Category and Month for 10 years, but line item detail for only the last 12 months. If the user needs to see line item detail for a prior year, they press a button that uses the "Open Qlikview Document" Action to open another qvw (document chaining) that has detail for that year.
The segmentation idea relies on being able to roll up to the smallest level of granularity required for long term analysis. I've found it will work in most cases, but you have to sit down and define the analysis scenarios with your users.
But you need to know how you want to do the segmentation or splitting the data model.