Note:
When a mouseDown event initiates for a text or numeric field, calculations or scripts run immediately. However, the value of the field does not change in response to calculations and scripts until the field loses focus. When a mouseDown event initiates for a signature field, the event initiates before the signature process begins.
You can use this event to perform an action as a direct response to a form filler clicking a button, or selecting a radio button or check box on a form. For example, you can use the mouseDown event for a check box to hide and show a field on the form. This event is conceptually similar to the click event and has a similar purpose.
See also 

mouseDown event