I see videos on this all the time, and they are sharing bad information that is leading to people getting cut. You do not hold knife straight up and down on the can while opening it. It pulls the jagged edge up, and one slip causes a nasty cut.
That is why it is important to hold the knife/ can opener at an angle to the side, all the force is being exerted away from the can. If it slips, your hand simply moves off to the side and not into the jagged edge. Here is another video showing the proper technique. This is showing a dedicated can opener instead of a old scout knife, but the can opener should operate the same way.