Open (or Gag) allows the mouthpiece to slide on the shank. Most bits are open. It is shown in the picture. Choose Gag in the drop down menu for normal, Gag action.
Ed Wright 3 Piece Twist LOCKED! Choose No Gag (also called Locked) if you don't want the mouthpiece to slide LONG: 8" Shank - 5 1/2 inch mouth MEDIUM: 7" Shank - 5 1/2 inch mouth SHORT: 5 1/2" Shank - 5 1/2 inch mouth