You bring the hurt just by rolling through an enemy's square. Once per Phase, you may make a free melee attack on an enemy as you move through their square immediately following their free attack on you (if any).
Note that your attack will happen in the square you are leaving from and your enemies attack will happen in the square they are in. If you are leaving a square occupied by another enemy, your action will be in a slagged square when you take it. An unconscious or stunned enemy doesn't provide a penalty (but they still provide a target!)