I probably used the wrong term calling it a "secondary sear". What I mean to describe is the method by which the hammer is stopped from following the bolt. There's an extra hook for that on an M16 hammer, allowing for the auto sear to hold the hammer til the bolt carrier whacks it. On the semi auto only the hammer is held by the disconnector, which then drops it onto the primary sear surface when the trigger is released.
Auto sear:
It just "fires the hammer" so to speak, and it's why even having a receiver with that extra hole drilled is a felony without a tax stamp, because it's such a simple little thing.
Then you add the rotating cam mechanism for 3 round burst and it gets even more weird
Here's another good resource that shows this.
http://www.madwulf.com/AR15vsM16.htm