How do I trap lateral bones that run down outside edge?

The instructions are specifically for flatfish, which generally have boney fins which run around the sides of the fish.

I think the instructions are telling you to use the blade of your knife to snag these and literally drag them away from the body.

A cooked flatfish is a very soft thing and you want to work delicately. There will be very little resistance when you drag the bones away so trapping them between your knife and the board beneath gives you enough grip to pull them gently away without messing with the fillet that you are trying to keep whole.