punches are just shapes moving through space. try to picture a right hand traveling on an axis thats invisibly connected to your head and continues to extend forever.
when you shoulder roll a right hand you're not trying to block the punch with your shoulder. you're trying to move your head off that axis.

your head is simply a point on an axis that continues to extend forever and the shoulder roll is just a movement we make to break away from that coordinate in space where any contact on the shoulder is actually totally incidental to that movement.

to shoulder roll a right hand: dip and twist on your back hip, your front shoulder will move up and swivel ahead of your back shoulder, and concurrently your back shoulder will move down and swivel behind your front shoulder, creating a break away action.

