For side release, the lanyard is attached to the blunt end and can be pulled out either by hand or by winch.
For rear release, the lanyard is attached to the tapered end and pulled from behind. The fid pries upon the trigger and stays captive with the shackle. It can also be used with no lanyard and operated as a 'handle' for convenient release. The plug fid is ideal for avoiding the need to crawl out into risky locations or for single-handed or night sailing.