That protuberance is to adjust the blade tension. You adjust the frame length so the blade will be held in both ends of the frame. You clamp the blade into the end where the handle is. You then make sure that the clamp on the other end will clamp the blade. You undo the one farthest from the handle and press the protuberance against a solid face so as to shorten the frame. While you have the frame compressed you make sure the blade is in the clamp and tighten the clamp. When you release the pressure on the frame the blade will be under tension and you will be able to cut a straight line. If there is no tension on the blade then you cannot control where the blade cuts. Depending on how much cutting you do you may have to readjust the blade tension as the blade heats up but make sure you release the blade after use as it maybe under too much tension once the blade cools and break the blade. If you pick the blade before you put tension on it the sound will be dull, once you have tension on it the sound will be bright, same as a guitar string.
Although what you are using it for it may help or it may not.