Open the Properties of each sheet. On the General tab you will discover an expression in the Conditional Show field.
The Main sheet will be visible only when no selections are made (variable vSelection = 0)
The Details sheet will be visible whenever a single value in field Car has been selected (variable vSelection = 1).
Who now is detecting selections made in field Car, and changing the value of variable vSelection, you may ask...
Open Settings->Document Properties->Triggers.
In the Field Event Triggers list, you will see a <has actions> indicator next to the Car field. If you select Car from this list, the OnSelect->Add Action(s) button changes to Edit Action(s), meaning that on every select in field Car, a trigger will be fired and the associated actions will be executed.
Click the button to see what actions are executed. There you'll find out who does what with vSelection.
