Your two questions are interrelated, the "corkscrew knob" controls the amount of pressure the foot places on the fabric while it is between the presser foot and the feed dogs. Two much pressure or too little is no good. The feed dogs should grab the fabric and pull it evenly under the presser foot. Righty-tighty - more pressure, lefty-loosey - less pressure. All you need to do with your hands is to gently guide the fabric under the foot.