I've read a few questions about turning a tenon and mounting wood with a tenon in scroll chuck jaws. So I took a couple pictures in hopes it would be easier to understand.
The first picture is of a round blank for a goblet with a tenon turned on one end. There is a good square solid shoulder for the jaws to contact.
The second is the wood in the jaws. The shoulder is tight against the jaws while the end of the tenon is just short of reaching the bottom of the jaws.
The jaws pull on the dovetail, pulling the shoulder tight into the jaws. With the shoulder tight against the jaws and the dovetail tight in the jaws it makes it hard for the wood to move or come loose.
Hope this helps those who have asked.
Sorry for the crappy pictures, me and my camera aren't very good at close ups.