Linch pins are a standard part used to secure loads, keep wheels on axles and many other tasks. The ring snaps down onto the pin to secure, and will snap open too, due to the ring being deformed as it's moved from one position to the other. Linch pins are also known as spring loaded retaining pins, spring loaded lynch pins, linch clips and safety pins.
Linch pins may also be used in place of cotter pins and R-clips, particularly when used for securing a tow bar hitch pin. They're commonly found on trailers, making them ideal for use in agricultural or marine industries.