BOL - Creating and Maintaining Databases - Full-Text Indexes

"An incremental population refreshes the full-text catalog by indexing data in full-text enabled columns with these characteristics:

-Rows that have been updated or inserted since the last population.
-Tables that have a timestamp column.
-All rows that have been enabled for full-text processing since the last population, or that have a schema that has been modified in any way since the last population"