Ben sees his original timeline as Desmond beats him up

Desmond sits in his car at the school and watches as Locke returns to work. As Locke begins to cross the parking lot, Desmond starts his car, but then Ben comes over and yells at him, screaming that he is the man who hit Mr. Locke.

As Desmond gets out of the car, Ben says that he's going to make a citizen's arrest. Desmond tells him not to get in his way and then punches Ben in the face. He then slams Ben repeatedly on the car's hood. Ben says that he won't let Desmond hurt Mr. Locke again, and Desmond says he's not trying to hurt him, he's here to help him let go (the same thing that Jack was trying to convince Locke to do by letting him perform the surgery).

Ben asks Desmond who he is and Desmond responds saying "You want to know who I am?" and then starts repeatedly punching Ben in the face. As he's being beaten, Ben sees flashes of the original timeline of Desmond beating him up at the marina when he was contemplating killing Penny.

Desmond then throws Ben on the ground, gets in his car and speeds away. Ben is left lying on the ground, his eyes wide after what he's just experienced.

So how does Desmond know exactly what will trigger each person's flashes? He seemed to have no interest in showing Ben his until he was confronted by him. Desmond has a specific plan, but we just don't know what it is.

Here we also see Ben's noble side again as he fearlessly stands up for John Locke, even as he's being beaten.