Why does the funnel-mantle locking apparatus exist, and how / why does it work?
Argonauts and other octopuses have a muscular mantle which is the outside of much of the body other than the arms.  A cavity is formed by the mantle in which the gills are located.  Water is pulled into the cavity through valve-like flaps and expelled through the tubular funnel (also called siphon or hyponom).  Here is a video of the flaps and funnel as an octopus breathes.

A typical octopus most often moves slowly by crawling across the sea floor.  They have a faster mode of locomotion, though, which is to quickly contract their mantle to force water out the funnel at high velocity.  This jet of water forces the body of the animal in the opposite direction. Here's a drawing of an argonaut moving with this mechanism:



(Here's a video of an octopus using its jet.)

As this jet propulsion is often used for escape, it is important for efficient movement that jet be directed so that the rounded mantle leads the movement and the arms trail.  Since the funnel is flexible, there is a need for a way to keep the funnel aimed away from the mantle.  The funnel–mantle locking apparatus exists for this reason.  A protrusion on the mantle fits into a slot on the funnel and while the animal is propelling itself holds the funnel in place.