1990 Bronco Automatic 4X4 Help????

I would like to know how exactly the push button 4x4 works? When I bought my bronco the front driveshaft was missing...... I am going to put a new one on but I am wondering why it is gone.... when I push the 4x4 button I hear a click but I don't thing the transfer case does anything, also what exactly tells the hubs to lock? The Button or the Transfer case IE would they lock even if the transfer case did not go into 4x4? Any help would be appreciated :)