The links-only mode renders the page titles quickly. No additional database access is necessary. Here is an example screenshot showing three suppressed transclusions:

If  properties-table mode is selected, additional document properties are rendered. This makes it easier for document authors to show interesting information, albeit the rendering takes a little longer. The properties selected are specified with the transclusion macro's Select Parameter.

The mode is only activated, when suppress-transclusion is set to true.