Pronged studs require a snap press and special die to install. Handheld pliers *cannot* install them.

 
Pronged studs are used to make double-sided snaps. So instead of having just 2 sides of a snap to form 1 connection, you can have another snap sandwiched in-between to form 2 connections. In the following example, a socket is installed with a pronged stud to form a double-sided snap (center).

You can actually have an infinite number of connections, depending on how many pronged studs you pile on top of each other.  The following example shows 6 snap connections.