There is a drop pin that locks the extension in, or out. Another version is a similar square tubing that simply folds alongside the tongue. Still another version just uses square U bolts to clamp to the underside of the existing tongue. There are lots of variation on the design depending on how much money you want to spend on labor welding, parts etc. The chapest is likely a box steel tube, with a ball coupler and a couple of square U bolts. It would be more labor intensive each time however, and if you are using the 'field and launc' method each time you go sailing throughout the season, this may get tedious. I think it cost me $75 to have a slide in tongue extender, and locking pin mechanism manufactured. That included the cost of the parts, excluding the ball coupler that was simply moved to the extension. Attached is another picture of the pin mechanism. That's a wheel jack and spare tire on the right. Pin is held in place by an upright tube so that the mecanism can be extended without the help of a second person. The pin is pulled up and rotated a 1/4 turn so that the flat steel seen rests on top of the tongue. The tongue extension is pulled out slightly, and the pin rotated back so that the handle is off to the side of the tongue. The extension is then pulled slowly out the rest of the way, and when the hole lines up with the pin, ... plunk... gravity drops the pin into the hole and locks the extension in place. When shortening the extension, the same procedure is used. The trailer wheels need to be blocked if you are using your vehicle to extend or retract the extension, but it can also be done by hand. I credit my welder with this design; I only thought of the upright tube to hold the pin in place. The chain simply keeps the pin from taking a walk. For highway trailing, I have a hole through the pin under the tongue that accepts a scissor clip so that the pin does not jump out at highway speeds.