[This is preliminary documentation and subject to change.]
Applications can invoke an accessible object's default action by calling its IAccessible::accDoDefaultAction method. You can query for an object's DefaultAction property by calling the IAccessible::get_accDefaultAction method.
Effectively, calling accDoDefaultAction causes the object to perform its primary function. For example, calling accDoDefaultAction for a button control would cause the button to be pressed as though the user had clicked it.