1. Use the View > Dual View command to open the database in two panes: the left and right panes.
2. With the current table as the "master" table in the left pane, select a desirable "slave/search" table in the right pane, then place the current selections in fields (columns) that are to be linked.
3. Use the Tools > Set Relation (Ctrl+=) command to turn the relation on and off.

The linked fields are displayed with the highlighted background color. Scrolling to another record in the "master" table in the left pane causes automatic filtering of the "search" table to find matching values in the linked field in the right pane.

Closing the dual view turns off the active relation.

Executing another the Tools > Set Relation (Ctrl+=) command turns off the active relation, no matter which tables are currently selected in the dual view.